These words refer to people who question or reject accepted beliefs, particularly religious dogma. The relatively formal sceptic and the relatively informal doubter are alike in emphasizing someone who questions or is not sure of a given belief. Doubter most often refers to uncertainty about a belief already put forward; this may be a body of religious dogma or any isolated, non-religious theory: missionaries whose purpose was to convert doubters and harrow sinners; a detailed investigation of the cause of death, with conclusions that should convince that most conscientious doubter . Sceptic can function in both these ways, but its main emphasis is on the questioning of accepted beliefs, perhaps not so much from a position of open-minded uncertainty as from one of an a priori conviction about where the truth lies: a confirmed sceptic about the value of any of the great religions. Most specifically, the word can refer to a philosophical belief that no final truths can be known, whether in any area of knowledge whatever or in some particular area: a sceptic about the validity of psychoanalytic theory. Less precisely, the word may refer to a person with a disengaged attitude of moral cynicism towards the worth or value of life as a whole: a sceptic who watched the fads and insanities of his time with an indifferent eye.

Freethinker and unbeliever are more closely tied to a rejection of religious belief than the previous pair. The first is an approving word, the second a disapproving word for the same sort of person, one who rejects the truth of a given religion or of all religions. Unbeliever might most often be used by a group of religious adherents to describe anyone not of their faith: a small, fanatical sect who regarded members of other religions as unbelievers doomed to suffer the torments of hell. More precisely, the word would indicate someone who belongs to no organized religion, or someone without religious beliefs of any sort; this sets it apart from doubter, which might indicate someone who belongs to a religion but is wavering in his convictions: addressing his sermons to doubters rather than to outright unbelievers . Freethinker emphasizes someone who has asserted the right to think and decide for himself about religious dogma; the word need not point to unbelief, but rather to a nonconforming, heterodox, approach that picks and chooses from one or many religions those things, if any, that seem worthy of belief: a freethinker who subscribed to the Sermon on the Mount, the Upanishads and the writings of Darwin, Freud and Lao-tzu; a freethinker who objected to the attitude of most religions towards women.

Agnostic relates roughly to sceptic and doubter , while atheist is more comparable to unbeliever ; both terms can be used in neutral description rather than in approval or disapproval. As widely used by the public press, political leaders and mass-circulation magazines, atheist is a contemptuous term, as witness its common coupling with communism: atheistic communism. Agnostic suggests someone who feels that no religious certainty is possible and that no proof or disproof of such a thing as the existence of God is valid: neither a believer nor unbeliever , but an agnostic . Atheist is the most specific of these words in being restricted to someone who does not believe in the existence of any sort of divinity: an atheist with a rigorous moral code based on the Ten Commandments.