Webster's Dictionary 
(1): (n.) The disbelief or denial of the existence of a God, or supreme intelligent Being.
(2): (n.) Godlessness.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
(from Ἄθιος , Without God), in popular language, means the negation of the existence of God.
1. Use Of The Word. — In all ages the term has been applied according to the popular conception of Θεός (God). Thus the word Ἄθεος , atheist, in old Greek usage, meant one who denied "the gods," especially the gods recognized by the law of the state. In this way several of the Greek philosophers (even Socrates) were called atheists (Cicero, Nat. Deorum, 1, 23). Cicero himself defines an atheist as one who in theory denies the existence of any God, — or practically refuses to worship any (Atheus, qui sine Deo est, impius, qui Deuma esse non credia, aut si credat, non colit, Deorum contemptor). This distinction of atheism into theoretical and practical has remained, in popular language, to this day. At a later period the Pagans applied the term atheists to the Christians as a generic name of reproach, because they denied the heathen gods and derided their worship (Eusebius, Ch. Hist. 4, 15; Bingham, Orig. Eccles. bk. 1, ch. 2, § 1). In the theological strifes of the early church it was not uncommon for the contending parties to call each other atheists, and, later still, the burning of heretics was justified by calling them atheists. The term was applied, in scientific theology, to such forms of unbelief as that of Pomponatius (Pomponazzi, † 1524) and Vanini ( † 1619). Bacon (Essay 16) uses the term to designate infidelity in general, and the denial of God in particular ("I had rather believe," he says, "all the fabulous tales in the Talmud and the Alcoran, than that the universal frame is without a mind"). So also in the De Augmentis (1, 11) he speaks of " ‘ a little knowledge inclining the mind of man to atheism." Toward the end of the 17th century the term is not unfrequently found, e.g. in Kortholt's De Tribus Impostoribus, 1680, to include Deism such as that of Hobbes, as well as blank Pantheism like Spinoza's, which more justly deserves the name. The same use is seen in Colerus's work against Spinoza, Arcana Atheismi Revelata. Tillotson (Serm. 1 on Atheism) and Bentley (Boyle Lectures) use the word more exactly, and the invention of the term deism induced in the writers of the 18th century a more limited and exact use of the word atheism. But in Germany, Reimannus (Historia Univ. Atheismi. 1725, p. 437 sq.) and Buddaeus (De Atheismo et Superstitione, 1723, ch. 3, § 2) use it most widely, and especially make it include disbelief of immortality (Farrar, Critical History of Free Thought, 414). Walch (Bibliotheca Theol. Selecta, 1757, 1:676, etc.) uses it to include Spinoza, Hobbes, and Collins as writers who, if not avowed atheists, are yet substantially such. It is a great mistake, in the interest of truth as well as in view of charity, to extend too far the application of the word atheist. Bayle does it (Bib. Crit.), also Brucker (Hist. Phil. t. i), both probably of design; and Harduin (Athei Detect. 1, Amsterd. 1733) puts Jansenius, Mialebranche, Quesnel, and others in his black list. On the other hand, it is both unwise and uncritical to except the extreme Pantheists (e.g. Spinoza) and Materialists from the number of Atheists. Lewes, in his Biographical History of Philosophy, and also in Fortnightly Review, 1866, p. 398, vindicates Spinoza from the charge of spiritual atheism, and states that Spinoza himself emphatically repudiated Atheism; but yet Lewes admits that logically there is little difference between Spinoza's Acosmism, which makes God the one universal being, and Atheism, which makes the cosmos the one universal existence. This point is fully discussed in Brenna, De gen. human. consensu in agnoscenda Divinitate (Florence, 1773, 2 vols. 4to). See also Perrone, Praelect. Theologicae (Paris, 1856, 1:238).
2. In scientific theology, atheism is opposed to Theism. The doctrine of Christian theism is that God is absolute, self-conscious personal spirit, the beneficent creator and upholder of the universe. Every system of philosophy or religion must be built upon this principle or its opposite; that is, must be either theistic or atheistic. Hence a great deal of what passes for Deism and Pantheism is in fact Atheism. Christianity apprehends God not as entirely apart from the world and exerting no providence (Deism), nor as existing only in the world (Pantheism), but as existing apart from creation, but himself creator and controller (i.e. Providence). On this theory of a living and personal God Christianity undertakes to explain the phenomena of the universe. Those who seek to explain these phenomena by substituting other ideas for this idea of God are, in the view of Christian theology, atheists.
The term should be applied to none who profess to believe in a personal, self-conscious, spiritual God. Atheism is divided into positive or dogmatic, which absolutely declares that there is no God, and negative or skeptical, which declares either (a) that, if there be a God, we cannot know either the fact or the nature of his existence, and therefore it is no concern of ours, or (b) that, if there be a God, we can only know of him by tradition or by faith, and can never have proof satisfactory to the intellect of his existence. Some Christian writers and philosophers have incautiously attempted to stand upon this latter ground. The so-called Positive Philosophy stands upon the first ground (a), but logically leads (in spite of Mr. J. S. Mill's denial, in his Exposition of Comte) to dogmatical atheism. To state that we only know, and only can know phenomena, is to exclude God; for God is not only no phenomenon, but is, in the Christian sense, the absolute ground of all phenomena. The theories which attempt to explain phenomena without the idea of God may be classed as (1) the Idealistic, which substitutes for the absolute, self-conscious Spirit, a so- called world-spirit; not a living, personal being, but an unconscious and abstract one — in a word, a mere conception of ideal being as the abstract totality of all individual conceptions; (2) the Materialistic, which substitutes far a personal God the forces inherent in matter, and holds that these sufficiently explain all phenomena; (3) the Subjective-idealistic, which asserts that phenomena are nothing but the creations or modifications of the thinking mind or subject, and that thought creates not only matter, so called, but God. To the first and third of these classes belong Fichte, Hegel, and (during his early life) Schelling, among the Germans, and their followers in England and America. To the second class belong Comte, and the socalled Positive philosophers in general. It is true that Lewes (Philosophy of the Sciences, p. 24) denies that Comte was an atheist; and Wallace (Art, Scenery, and Philosophy in Europe), while admitting Comte's individual atheism, denies that atheism is a characteristic of Positivism. But these denials are vain, so long as the very aim of the Positive method is to eliminate mind and will from the universe. A science of pure phenomenalism can never coexist with Christian theism. Perhaps the most open declarations of atheism in modern times are to be found in D'Holbach's Systreme de la Nature (1770), the ultimate fruit, in atheistic materialism, of the sensational philosophy. Even Voltaire pronounced it "abominable" (see note to Brougham, Discourse on Natural Theology; Renouvier, Philosophie Moderne, bk. 5, § 2).
The doctrine of the book is that nothing, in fact, exists but matter and motion, which are inseparable. "If matter is at rest, it is only because hindered in motion, for in its essence it is not a dead mass. Motion is twofold, attraction and repulsion, and the different motions we see are the products of these two; and through these arise the different connections and the whole manifoldness of things, under laws which are eternal and unchangeable. It flows from these positions, first, that man is material, and, secondly, that the belief in God is a chimera. Another chimera, the belief in the being of a God, is the twofold division of man into body and soul. This belief arises like the hypothesis of a soul- substance, because mind is falsely divided from matter, and nature is thus made twofold. The evil which men experienced, and whose natural cause they could not discover, they assigned to a deity which they imagined for the purpose. The first notions of a God have their source, therefore, in sorrow, fear, and uncertainty. We tremble because our forefathers for thousands of years have done the same. This circumstance awakens no auspicious prepossession. But not only the rude, but also the theological idea of God is worthless, for it explains no phenomena of nature. It is, moreover, full of absurdities; for since it ascribes moral attributes to God, it renders him human; while, on the other hand, by a mass of negative attributes, it seeks to distinguish him absolutely from every other human being. The true system, the system of nature, is hence atheistic. But such a doctrine requires a culture and a courage which neither all men nor most men possess. If we understand by the word atheist one who considers only dead matter, or who designates the moving power in nature with the name God, then is there no atheist, or whoever would be one is a fool. But if the word means one who denies the existence of a spiritual being, a being whose attributes can only be a source of annoyance to men, then are there indeed atheists, and there would be more of them, if a correct knowledge of nature and a sound reason were more widely diffused. But if atheism is true, then should it be diffused.
There are, indeed, many who have cast off the yoke of religion, who nevertheless think it is necessary for the common people in order to keep them within proper limits. But this is just as if we should determine to give a man poison lest he should abuse his strength. Every kind of Deism leads necessarily to superstition, since it is not possible to continue on the stand-point of pure Deism. With such premises the freedom and immortality of the soul both disappear. Man, like every other substance in nature, is a link in the chain of necessary connection, a blind instrument in the hands of necessity. If any thing should be endowed with self-motion, that is, with a capacity to produce motion without any other cause, then would it have the power to destroy motion in the universe; but this is contrary to the conception of the universe, which is only an endless series of hincessary motions spreading out into wider circles continually. The claim of an individual immortality is absurd. For to affirm that the soul exists after the destruction of the body, is to affirm that a modification of a substance can exist after the substance itself has disappeared. There is no other immortality than the remembrance of posterity" (Schwegler, History of Philosophy, § 32). The Dictionnaire des Athees of Sylvain Marechal, edited by Lalande (Paris, 1799), is a flagrant specimen of the same kind. The strongest German development is Strauss's identification of God with the universal being of man, in his Dogmatik; and Feuerbach's bald atheism, in his Wesen des Chr7istenthums (Smith's Hagenbach, History of Doctrines, § 293). The so-called English ‘ "secularism" is an atheistic doctrine resting on, or similar to that of the Positive Philosophy. It holds the eternity of matter; it knows of nothing greater than nature; its creed is a stern fatalism; its worship is labor; its religion is science; its future is — a "black, impenetrable curtain."
One of its advocates says, "A deep silence reigns behind the curtain; no one within will answer those he has left without; all that you can hear is a hollow echo of your question, as if you shouted into a cavern" (Holyoake, Logic of Death). Such is the wretched atheism which is expounded by itinerant lecturers, and disseminated by periodical pamphlets throughout the length and breadth of Great Britain, and which is perverting and contaminating the minds of the more thoughtful and inquisitive among the working classes of that country to an unprecedented and incredible extent (London Review, 11, 20. See also Christian Examiner, Boston, Nov. 1859; North British Review, Nov. 1860). We close this article with the following admirable passage from a modern writer: "The whole history of philosophy and theology shows that, when the material world is taken by itself, it is a contradiction of God. Atheism was not coeval with man. No atheist pretends that it was. It was always a denial, and a denial presupposes an affirmation. The denial of a God presupposes the existence in man of some faculty anterior to reflection which may apprehend Infinite Being. It is a denial, also, which has always been preceded by misapprehension of God. Pseudo-theism precedes atheism. The first denial of God is made unintentionally. Men begin to worship remarkable peculiarities of the material universe. Thus worship fell from its primitive spirit and truth into deification of the heavens and earth, to which the overflowing soul of man lent some of its own unbounded life. The Book of Job, one of the oldest of human writings, refers to this primitive idolatry in the following words: ‘ If I beheld the sun when it shined, or the moon walking in brightness, and my heart hath been secretly enticed, or my mouth hath kissed my hand, this also were an iniquity to be punished by the judge; for I should have denied the God that is above.'
This declaration plainly shows that such things had begun to be in his day, but were not universal. It is a very simple exposition of the rise of idolatry everywhere. Pseudo-theism is incipient atheism; but it testifies to a pure theism going before it. The mistake of this early false worship is, as every one sees, the radical mistake of materializing the conception of God. It is the result of idly resting in an impression made by material objects. This impression would never have been made unless those objects expressed a life corresponding to ours. It was an impression at first perhaps innocently cherished as a religious influence; but it proved the means of shutting out God, the Being of love, wisdom, and power, as an object of true faith, and the source of a glowing worship. It ended in atheism. In modern times, the same result has followed from men's seizing on the external as their means of making clear the Divine Life. It would be quite possible to trace a parallel between the consequences of giving the great name of God to the sun, moon, and earth, and the consequences of giving the same august name to laws of nature which are simple categories of the human understanding; for the forms of the understanding may stand between the soul and God, preventing his immanence in the consciousness, no less than the stars of heaven and the imposing forms of earth. The forms of the understanding, though impalpable, are media, no less than visible and palpable matter; and it is important to observe that they are as much so.
They have proved as fruitful sources of atheism when rested on as ultimate; for if they have not corrupted man's sensual nature by making his rites of worship bodily vice, they have paralyzed his spirit by substituting intellectual speculation for the fervent spiritual exercise which involves his might and heart, no less than his mind, in a reasonable service. But to give a logical priority of matter to mind, in an argument for the being of the spiritual God, is to beg the question at once. This Plato has observed. He says in his Laws: ‘ Atheists make the assumption that fire and water, earth and air, stand first in the order of existences, and calling them nature, they evolve soul out of them. In scrutinizing this position of the class of men who busy themselves with physical investigations, it will perhaps appear that those who come to conclusions so different from ours, and irreverent of God, follow an erroneous method. The cause of production and dissolution, which is the mind, they make, not a primary, but a secondary existence" (Christian Examiner, Sept. 1858). (See Infidelity); (See Materialism); (See Pantheism); (See Theism). See also, besides the authors cited in the course of this article, Buddaeus, Theses De Atheismo (Jena, 1717; in German, 1723); Heidenreich, Briefe Ib. D. Atheismus (Leipzig, 1796); Reimann, Historia Atheismi (Hildesh. 1725); Stapfer, Instit. Theol. Polem. vol. 2, ch. 6; Doddridge, Lectures On Pneumatology, etc., Lect. 33; Cudworth, Intellectual System, bk. 1, ch. 3; Buchanan, Modern Atheism, Under Its ‘ Forms Of Pantheism, Secularism, Development, And Natural Laws (Boston, 1859, 12mo); Gioberti, L'Etude De La Philosophie, 2. 105; Thompson, Christian Theism (N. Y. 1855,12mo); Tulloch, Theism (N. Y. 1855, 12mo); Morell- History of Modern Philosophy; Constant, De la Religion, 3, 20; New American Cyclopedia, s.v.; Herzog, Real-Encyklopadie, 1, 577; Bartholmess, Hist. Crit. des .Doctrines de la Philosophie Moderne, bk. 13; Farrar, Critical History of Free Thought, chap. 7; Pearson, Modern Infidelity, chap. 1, and Appendix; Chalmers, Institutes of Theology, book 1, chap. 3; Riddle, Bampton Lecture, 1862, Lecture in; Van Mildert, Boyle Lectures (London, 1820, 2 vols. 8vo); Watson, Theological Institutes, part 2, chap. i.
International Standard Bible Encyclopedia 
ā´thē̇ - iz ' m ( ἄθεος , átheos , "without God" ( Ephesians 2:12 )): Ordinarily this word is interpreted to mean a denial of the existence of God, a disbelief in God, the opposite of theism. But it seems better that we should consider it under four heads, in order to obtain a clear idea of the different meanings in which it has been used.
(1) The Classical
In this sense it does not mean a denial of the existence of a Divine Being, but the denial of the existence or reality of the god of a particular nation. Thus the Christians were repeatedly charged with atheism, because of their disbelief in the gods of heathenism. It was not charged that they did not believe in any god, but that they denied the existence and reality of the gods worshipped, and before whom the nation hitherto had bowed. This was considered so great a crime, so dangerous a thing to the nation, that it was felt to be a just cause for most cruel and determined persecutions. Socrates' teaching cast a shadow on the reality of the existence of the gods, and this charge was brought against him by his contemporaries. Cicero also uses the word in this sense in his charge against Diagoras of Athens. Indeed, such use of it is common in all classical literature.
It is not meant that the various philosophic systems to which this term is applied actually deny the existence of a Divine Being or of a First Cause, but that they are atheistic in their teaching, and tend to unsettle the faith of mankind in the existence of God. There is indeed a belief in a first cause, in force, in motion, in a certain aggregation of materials producing life, but the Divine Being as taught by theism is absolutely denied. This is true of the Idealism of Fichte, of the Ideal Pantheism of Spinoza, the Natural Pantheism of Schelling, and similar forms of thought. In applying the word atheism to the teaching here given, theism does not intend to assail them as wholly without a belief in a Divine Being; but it affirms that God is a person, a self-conscious Being, not merely a first cause or force. To deny this fundamental affirmation of theism is to make the teaching atheistic, a denial of that which is essential to theism ( Hebrews 11:3 ).
It absolutely denies the existence of God. It has often been held that this is, in fact, impossible. Cousin has said, "It is impossible, because the existence of God is implied in every assertion." It is true, however, that in all ages there have been persons who declared themselves absolute atheists. Especially is this true of the 18th century a period of widespread skepticism - when not a few, particularly in France, professed themselves atheists. In many cases, however, it resulted from a loose use of the word, careless definition, and sometimes from the spirit of boastfulness.
(4) Practical Atheism
It has nothing at all to do with belief. Indeed it accepts the affirmations of theism. It has reference wholly to the mode of life. It is to live as though there was no God. It takes the form often of complete indifference to the claims of the Divine Being or again of outbroken and defiant wickedness ( Psalm 14:1 ). That this form of atheism is widely prevalent is well known. It is accompanied in many cases with some form of unbelief or prejudice or false opinion of the church or Christianity. Dogmatic atheism is no longer a menace or even a hindrance to the progress of Christianity, but practical atheism is widespread in its influence and a dangerous element in our modern life (compare Isaiah 31:1; Jeremiah 2:13 , Jeremiah 2:17 , Jeremiah 2:18; Jeremiah 18:13-15 ). Whatever the form, whether it be that of religious agnosticism, denying that we can know that God exists, or critical atheism, denying that the evidence to prove His existence is sufficient, or dogmatic, or practical atheism, it is always a system of negation and as such tears down and destroys. It destroys the faith upon which all human relations are built. Since there is no God, there is no right nor wrong, and human action is neither good nor bad, but convenient or inconvenient. It leaves human society without a basis for order and human government without foundation (Rom 1:10-32). All is hopeless, all is wretchedness, all is tending to the grave and the grave ends all.
Arguments against atheism may be summarized as follows: (1) It is contrary to reason. History has shown again and again how impossible it is to bring the mind to rest in this doctrine. Although Buddhism is atheistic in its teaching, idolatry is widespread in the lands where it prevails. While the Positive Philosophy of Auguste Comte was based on a denial of the existence of God, his attempt to found the new religion of humanity with rites and ceremonies of worship reveals how the longing for worship cannot be suppressed. It is a revelation of the fact so often seen in the history of human thought, that the mind cannot rest in the tenets of atheism.
(2) It is contrary to human experience. All history testifies that there are deep religious instincts within the human breast. To regard these as deceptive and unreasonable would itself be utterly unreasonable and unscientific. But the fact of such spiritual longing implies also that there is a Being who is responsive to and can satisfy the cry of the heart ( Hebrews 11:6 ). In his Bampton Lectures Reville has said on this subject: "It would be irrational in the last degree to lay down the existence of such a need and such a tendency, and yet believe that the need corresponds to nothing, that the tendency has no goal."
(3) It fails to account for the evidence of design in the universe. See Cosmology .
(4) It fails to account for the existence of man and the world in general. Here is the universe: how did it come to be? Here is man: how is he to be accounted for? To these and like questions, atheism and atheistic philosophy have no adequate answer to give. See also Cosmology; Creation; God .
The Nuttall Encyclopedia 
Disbelief in the existence of God, which may be either theoretical, in the intellect, or practical, in the life, the latter the more common and the more fatal form of it.