From BiblePortal Wikipedia

Webster's Dictionary [1]

etc. See Skeptic, Skeptical, Skepticism, etc.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

(from Gr. Σκέπτομαι , I Consider ) strictly denotes that condition in which the mind is before it has arrived at conclusive opinions when it is still in the act of investigating or reflecting. Scepticism is therefore the opposite of dogmatism. Disbelief is quite a secondary meaning of the term. The Sceptics (disciples of Pyrrho of Elis) aimed at an undisturbed tranquillity of mind, to be attained by a constant balancing of opposing arguments, thus reducing everything to a state of uncertainty and doubt. Popularly, the word is employed to signify the rejection of all religion infidelity.

Scepticism has assumed several forms, of which the following are among the most common.

(1) Pantheism, or antisupernaturalism. Spinoza, the leader of this class, talks of nothing less than demonstration, and of being infallibly led to each conclusion by arguments which admit of no reply; a geometrical method of demonstration, the use of which, he said, made it unnecessary to attend to the arguments of opponents.

(2) The Academic form, which originated with the Sophists, and which Bayle revived, the essence of which consists in opposing all the systems of speculative belief to each other. Academic doubt is ever seeking, for the avowed purpose of never finding; and perpetually reasoning, in order that it may never come to any conclusion.

(3) The Absolute form, which strikes at the root of all opinions, and appears to found a system of universal doubt in the human understanding itself. Of this kind of scepticism the writings of Hume furnish the great and unrivalled example in modern times.

(4) Ridicule. This contains no philosophy, but is a mere series of doubting and jesting. Such was the scepticism of Voltaire.

(5) The Historical form: this is contained in a narrative relating to the times and circumstances with which religion is chiefly concerned; and while preserving an outward regard to morals, misrepresents with irony the miraculous history of the Bible, and takes care, without absolutely falsifying facts, to place it in an absurd and improbable point of view. The history of Gibbon, dealing much in insinuation and very little in argument, is, perhaps, the most dangerous production in this class which has yet appeared, because it least admits of a reply. For who, as Paley observes, "can refute a sneer?"

(6) Sentimental infidelity. Such was the unbelief of Rousseau. Other infidels would destroy Christianity without having fixed on any other system to substitute in its place; but, if Rousseau has no system, he has abundance of "sentiments" and imaginations, and has a dim poetical deity of his own to worship, though he can assign no definite attributes to it, nor form any positive conception of his shadowy god. The most modern form of scepticism is rationalism (q.v.), which strictly signifies that method of thought which, in matters of religion, not only allows the use of reason, but considers it indispensable. The term has now, however, acquired a wider meaning, and stands in opposition to supernaturalism (q.v.), or the belief in that which transcends, or, as others view it, contradicts both nature and reason as, for example, miracles.

The Nuttall Encyclopedia [3]

Primarily doubt respecting, and ultimately disbelief in, the reality of the super-sensible, or the transcendental, or the validity of the evidence on which the belief in it is founded, such as reason or revelation, and in religious matters is tantamount to infidelity more or less sweeping.