English Synonyms and Antonyms, by James Champlin Fernald
The skeptic doubts divine revelation; the disbeliever and the unbeliever reject it, the disbeliever with more of intellectual dissent, the unbeliever (in the common acceptation) with indifference or with opposition of heart as well as of intellect. Infidel is an opprobrious term that might once almost have been said to be geographical in its range. The Crusaders called all Mohammedans infidels, and were so called by them in return; the word is commonly applied to any decided opponent of an accepted religion. The atheist denies that there is a God; the deist admits the existence of God, but denies that the Christian Scriptures are a revelation from him; the agnostic denies either that we do know or that we can know whether there is a God.