Hastings' Dictionary of the New Testament 
Monotheism —At whatever period in their early history the people of Israel may be supposed to have passed through the obscure and uncertain stages of belief that precede a clear and reasoned theism, that period had been left behind long before the days of Christ and the NT writers. The bitter experiences of exile and suffering on the one hand, and on the other the lofty teachings of prophets and men of God, had eradicated all tendencies to polytheism, and had fixed immovably in the conscience and conviction of the entire nation the faith that Jehovah was the one God of the whole earth. If Israel’s early beliefs, as some contend, were henotheistic, and conceded a place and right to other national gods, as Chemosh, Molech, or Rimmon, as equal and paramount lords of their own peoples, such recognition of external divinities had long since ceased to be permissible. There were not really gods many and lords many; there is one God the Father, and one Lord Jesus Christ ( 1 Corinthians 8:6).
This monotheistic belief, however, is assumed rather than formulated or defined in the Gospels. The doctrine that God is one, universally supreme and without rival, does not need to be explained or defended, for it runs no risk of being assailed. Like the belief in the existence of God, it is an article of faith accepted on all sides, by Jesus and by His opponents, and is rather implicit in the thought than explicit in the teaching of Christ and of His disciples.
While, however, this is true, and all the more so because His controversy with the Jews turned largely upon the question of His claim to equality with God, and the blasphemy which this claim appeared to them to imply, epithets and phrases may readily be quoted from the Gospels which have no meaning except as presupposing an absolute and pure monotheism. Such phrases, as would naturally be anticipated, are more generally employed by St. John than by the Synoptists. Thus the Prologue of the Fourth Gospel, tracing all things back to God with whom the Word is one ( John 1:1), asserts nothing less than the uniqueness as well as the eternity and sovereignty of Him from whom they proceed; and the true Light entering into the world enlighteneth not this or that nation only, but every man ( John 1:9). To the same effect and with the same background of accepted and common belief are the repeated declarations of His oneness with the Father ( John 10:30; John 10:38; John 14:10; cf. John 17:21; cf. John 17:23). The area and claims of the Divine Kingdom, the Kingdom of God, are explicitly enlarged beyond any mere national limits, and made to embrace the whole world ( Luke 16:16, John 4:21 ff.), and so the disciples are taught to pray that it may come upon earth, as it is in heaven ( Matthew 6:10). It is indeed not bodily or material ( Luke 17:21), but transcends the world ( John 18:36). In the Last Judgment, again, all nations are gathered before the throne, and all receive sentence. ‘The field’ in which the seed is sown is ‘the world’ ( Matthew 13:38); and the final injunction to Christ’s followers is that they are to go into all the world to make disciples of all the nations ( Matthew 28:19).
The same teaching is conveyed with more or less directness in the assertion of the subordination and judgment of the prince of this world ( John 16:11); in the stress laid upon the unique obligation and importance of love to God as constituting the first and greatest commandment ( Matthew 22:37 || Mark 12:30, Luke 10:27); in the appeal made by Christ Himself to a similar unique obligation of worship and service to the one only God ( Matthew 4:10 || Luke 4:8); in the emphatic affirmation of a common Fatherhood and Godhead ( John 20:17; cf. John 8:41); and in the solemn declaration of the permanence and inviolability of the words of the Son ( Matthew 24:35 || Mark 13:31, Luke 21:33), while elsewhere there is ascribed to Him that omniscience which is an attribute of God Himself ( John 16:30).
There are also passages in which the epithet ‘one’ or ‘only’ is directly applied to the Divine Ruler, thus claiming for Him with more or less emphasis the sole dominion and the exclusive right to homage. ‘The Lord our God is one Lord’ ( Mark 12:29 from Deuteronomy 6:4, cf. Mark 12:32). The God who forgives sins is εἶς ( Mark 2:7), or μόνος ( Luke 5:21); He is unique in goodness ( Matthew 19:17 || Mark 10:18, Luke 18:19); the sole Father ( Matthew 23:9); and the only God ( John 5:44).
Some of these expressions might, it is true, be satisfied by a wide conception, such as the ancient prophets had formed, of a God of Israel to whom the sons of Israel were a first interest and charge, or even of a Sovereign the limits of whose sway left room for other sovereigns beside Him. Not all of them, evidently, if read apart and by themselves, will bear the weight of a full monotheistic inference. Taken together, however, and in their context, their joint and several significance is unmistakable. They assume on the part of speaker and hearer alike a belief in the sole supremacy of one God. Nor is this inference as to their meaning seriously contested.
Moreover, in one passage ( John 17:3) there is found a perfectly distinct and unequivocal assertion of monotheistic doctrine; eternal life is to gain a knowledge of the only true God (τὸν μόνον ἁληθινὸν θεόν). Other phrases, in themselves less definite or comprehensive, must clearly be received and interpreted in the light of this, if an adequate conception of Christ’s teaching concerning the Father is to be reached. The principle is applicable to other elements of His instruction than that under consideration. The whole is to be construed and expounded by means of the loftiest and most comprehensive statements of doctrine, not to be attenuated to those which may be more particular or obscure.
The conclusion, therefore, is that a monotheistic belief is everywhere assumed in the Gospels; and if it is rarely formulated, the reason is to be sought in the universal assent with which it was received. Christ did not need to teach with definiteness and reiteration, as though it were a new truth, that there is one only Lord of heaven and of earth; for this belief was common to Himself and to His hearers, and formed the solid and accepted foundation of their religious faith.
Literature.—Treatises on the Theology of the NT discuss the conception of God, and the general doctrine is treated in works on Theism; cf. Ed. Caird, Evolution of Religion 2 [Note: designates the particular edition of the work referred] , 2 vols., Glasgow, 1894; Orr, Christian View of God and the World 1 [Note: designates the particular edition of the work referred] , pp. 91–96.
A. S. Geden.
Holman Bible Dictionary 
Commandments shall not embrace any of those other gods as gods who compete for the loyalty of the people. The Lord who brought Israel out of the land of Egypt will allow no compromise in the loyalty of the people. That assertion assumes the existence of other false gods who could call for loyalty and commitment from the Lord's people. That kind of belief system is commonly called henotheism.
In contrast to the call for strict commitment to the Lord alone, to a kind of divine jealousy that would tolerate no commitments from the people to gods other than the Lord, even though other gods might tempt the Lord's people with offers of power, the people among whom Israel lived in the early years of occupation in Canaan believed in numerous gods whose activities influenced their lives. Principal among the gods of the Canaanite pantheon were the great father figure, El; the younger hero, Baal; the adversary against order in the created land, Yam; the consort for Baal, Anat; and the ruler of Sheol, the place of the dead, Mot. In the Canaanite story about the various events involving these gods, Baal and his consort were primarily responsible for the success or failure of the agriculture in the social structure of Canaan. The fertility of the land depended on the fertility of Baal and his consort. The cult for the Canaanite farmers sought to stimulate the fertility of the divine couple, and thus the fertility of the land, by participating in fertility rituals at central sanctuaries called high places. The sexual activities of these rituals would stimulate Baal and his consort to similar activities and thus secure the fertility of the land.
One particular phase of that cult developed its drama from a belief that in the fall of the year, the time when vegetation on the earth dies, Baal died and descended into Sheol. On hearing the news of this tragedy, Anat began a long search for Baal. She found him in Sheol and effected his resurrection from the dead by coaxing him back to activity in the world of the living. This scene of resurrection occurred in the spring when the world springs back to life. Such mythology undergirds a belief system that depended on the activities, indeed, the interrelationship, of many gods. That system can be called polytheism.
A move away from henotheism and polytheism appears first in the Old Testament among the prophets. The prophetic movement appears as early as the prophet Elijah. Competition between the people of Israel and the people of Phoenicia was highlighted by a competition for loyalty of the people between the Lord and Baal. That competition came to its sharpest focus in the story about the contest between Elijah, the prophet for the Lord, and the prophets of Baal on Mount Carmel ( 1 Kings 18:1 ). The issue for the contests is still competition for the loyalty of the people. That issue focused on the question of genuine claim to status as God. “If the Lord be God, follow him: but if Baal, then follow him” ( 1 Kings 18:21 ). The issue of claim to genuine status as God is then focused on power. “The God that answereth by fire, let him be God” ( 1 Kings 18:24 ).
The pressure of the Exile challenged Yahweh's claim as the only God. If the Lord is really God and if that claim can be substantiated by acts of power, then how could the people of the Lord lose their independence and their land to a foreign people? Would the success of the Babylonians against Judah not undergird the claim that Marduk, the god of the Babylonians, is really God? Would it not suggest that the Lord, the God of the Judeans, had been defeated by Marduk, the god of the Babylonians? The prophets' response to this crisis was: the tragedy of the Exile was not the result of the power of Marduk against the power of the Lord, a result that would establish Marduk as God. To the contrary, the tragedy of the Exile was the result of Israel's own God using the Babylonians as an instrument of punishment against the Lord's own people since they had violated the terms of the covenant that bound them together. That theological justification for the Exile (see Amos 2:4-8 ) opened the door for a theological, philosophical position that asserted the existence of only one God who is Lord not only of Israel but also of all the rest of the world. That position can be called monotheism.
The beautiful poetry of Isaiah 40-66 represents the height of Israel's monotheism. For the first time in the Old Testament literature, a prophet explicitly argued that no other gods exist. The Lord alone is God. “I am the Lord, and there is none else, there is no God beside me: I girded thee, though thou hast not known me: That they may know from the rising of the sun, and from the west, that there is none beside me, I am the Lord, and there is none else ( Isaiah 45:5-7 ).” With that poetry, Israel reached a fully developed monotheism. Moreover, such monotheism asserts that the only God is Creator of the world: “I am the Lord that maketh all things; that stretcheth forth the heavens alone” ( Isaiah 44:24 ) and its Savior and Redeemer: “I, even I, am the Lord, and beside me there is no savior.” ( Isaiah 43:11 ).
George W. Coats
Webster's Dictionary 
(n.) The doctrine or belief that there is but one God.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
(from Μόνος , One, and Θεός , God) is the belief in and worship of one only God, in opposition to polytheism, which acknowledges a plurality of gods. All the different mythologies have, among the host of gods with which they people heaven and earth, some, superior or supreme deity, more or less defined, but in every case distinguished above the others; and in the history of all the different nations where polytheism has obtained we may trace a period when the idea of one God was more or less prevalent. The most ancient traditions concur with the testimony of sacred Scripture in representing this as the primary and uncorrupted religion of mankind. M. Renan, in his Histoire Generale Et Systeme Compare Des Langues Semitiques (Par. 1858, 2d ed.), and Nouvelles Considerations Sur Le Caractere General Des Peuples Semitiques Et En Particulier Sur Leur Tendance Au Monotheisme (Par. 1859), takes the ground that the Shemitic nations of the world are the propagators of the doctrine of the unity of God — indeed, that "of all the races of mankind, the Shemitic race alone was endowed with the instinct of monotheism... a religious instinct analogous to the instinct which led each race to the formation of its own language" (page 73). Max Miller, however, takes exception to this position, and insists upon it that the primitive intuition of God was in itself neither monotheistic nor polytheistic, but consisted solely in that simplest article of faith — that God is God. "This must have been the faith of the ancestors of mankind previously to any division of race or confusion of tongues... It is too often forgotten by those who believe that a polytheistic worship was the most natural unfolding of religious life, that polytheism must everywhere have been preceded by a more or less conscious theism. In no language does the plural exist before the singular. No human mind could have conceived the idea of gods without having previously conceived the idea of a god... There are, however, in reality two kinds of oneness which, when we enter into metaphysical discussions, must be carefully distinguished, and which for practical purposes are well kept separate by the definite and indefinite articles... If an expression had been given to that primitive intuition of the Deity, which is the mainspring of all later religion, it would have been, 'There is a God,' but not yet 'There is but one God.' The latter form of faith, the belief in one God, is properly called monotheism, whereas the term henotheism would best express the faith in a single God" (Chips, 1:348-50). This kind of monotheism, according to Miller, "forms the birthright of every human being... In some form or other, the feeling of dependence on a higher power breaks through in all the religions of the world, and explains to us the meaning of St. Paul, ‘ that God, though in times past he suffered all nations to walk in their own ways, nevertheless left not himself without witness, in that he did good, and gave us rain from heaven and fruitful seasons, filling our hearts with food and gladness.' This primitive intuition of God, and this ineradicable feeling of dependence on God, could only have been the result of a primitive revelation, in the truest sense of that word" (pages 346-8, see also pages 363, 374; comp. Gould, Origin of Religious Belief, 1:267277). In this respect Judaism, Christianity, and Mohammedanism agree.
"Two facts," says Gould, "arrest our attention... the prevalence of monotheism, and the tendency of civilization towards it. Monotheism is at present the creed of a large section of the human race. The Christian, the Jew, and the Mohammedan hold the unity of the great cause with varying distinctness, according to their powers of abstraction" (Origin of Religious Belief, 1:238). But in regard to the Trinity they seriously differ, the Mohammedan and the Jew rejecting with vehemence the least approach to a trinitarian conception of the Deity. "The monotheism of the Mohammedan," says J.F. Clarke, "is that which makes of God pure will; that is, which exaggerates personality (since personality is in will), making the divine One an infinite Free Will or an infinite I. But will divorced from reason and love is wilfulness, or a purely arbitrary will. The monotheism of the Jews differed from this in that it combined with the idea of will the idea of justice. God not only does what he chooses, but he chooses to do only what is right. Righteousness is an attribute of God, with which the Jewish books are saturated. Both of these systems leave God outside of the world; above all as its Creator and Ruler, above all as its Judge; but not through all and in all. The idea of an infinite love must be added and made supreme, in order to give us a Being who is not only above all, but also through all and in all. This is the Christian monotheism... Mohammed teaches a God above us; Moses teaches a God above us, and yet with us; Jesus teaches God above us, God with us, and God in us" (Ten Great Religions, pages 481-83). See Jahrb. deutsch. Theol. (1860), 4:669; Brit. Quar. Rev. (April 1873), art. 2; Lond. Quar. Rev. volume 127. (See Unity Of God).
Gould holds to a gradual development of monotheism. Recognising a Jewish, Mohammedan, and Christian monotheism. he traces first the development of the Jewish, which, under Moses, received "its final and complete form as a system, and embraced four leading doctrines:
(1) the absolute being of God;
(2) the absolute unity of his being;
(3) the difference in kind of matter from God;
(4) the subjection of matter to God"
(1:262; comp. (See Mosaism) ). The Mohammedan's monotheism he recognises as "the offspring of Jewish monotheism." Yet has the pure deism proved inferior to the Jewish, for "as a working system it annihilates morality. Before the almighty power of God the creature is nothing. Man, ox, ass, are on a level; and if the notion be humbling to him, he may recover a little self-respect when he remembers that the archangels are in no better plight. Between man and God is a profound and wide abyss, and no bridge spans it. Too far above man to sympathize in any way with him, God can yet crush him with his jealousy. If man attempt to attribute to himself anything that is of God, and appear to encroach on his all. engrossing majesty by ever so little, the wrath of God is kindled and man is levelled with the dust" (1:265). "It is," says Palgrave, "his singular satisfaction to let created beings continually feel that they are nothing else than his slaves, tools, and contemptible tools also, that thus they may the better acknowledge his superiority, and know his power to be above their power, his cunning above their cunning, his will above their will, his pride above their pride; or, rather, that there is no power, cunning, will, or pride save his own. But he himself, in his inaccessible height, neither loving aught save his own and self-measured decree, without son, companion, or counsellor, is no less barren for himself than for his creatures, and his own barrenness and lone egoism in himself is the cause and rule of his indifferent and unregarding despotism around" (Arabia, 1:366). (See Polytheism).
Christian monotheism Gould excludes from comparison with the Jewish and Mohammedan, because "its doctrine of the Trinity and the incarnation remove it from the class to which which Mosaism and Islamism... belong" (1:277). (See God); (See Trinity). See besides Gould, Clarke, Max Miller, and Renan; Hagenbach, Hist. Of Doctrines, 1:330; Christlieb, Modern Doubt And Christian Belief (N.Y. 1875, 8vo), lect. 3 and 4; Lewes, Hist. Philos. volume 2 (see Index); Liddon, Divinity Of Christ, pages 67, 76, 95, 270, 307; and the literature appended to the article THEISM (See Theism) .
The Nuttall Encyclopedia 
Belief in the existence of one God, or the divine unity, or that the Divine Being, whether twofold, as in dualism, threefold, as in Trinitarianism, is in essence and in manifestation one.