Agnostic, Deist, Freethinker, Unbeliever and Infidel all designate a person who rejects some or all of the essential doctrines of religion and particularly the existence of God.
An atheist is one who denies the existence of God; an agnostic is one who withholds belief (though he may not deny the possible existence of a supreme being) because he does not know and is unwilling to accept as proof the evidence of revelation and spiritual experience; a deist is one who rejects the conception of a supreme being as ruler and guide of men and the universe, but still believes in a god who is the creator and the final judge of men.
Since deism implies a denial of revelation and supernaturalism, deist has often been used as though it were the equivalent of atheist. Freethinker suggests loss of faith and the rejection of any or all of the tenets of revealed religion in favor of what seems rational or credible.
Unbeliever is more negative than freethinker, because it carries no implication of a substitute for faith.
Infidel denotes one who is not a Christian or who opposes Christianity; it is used by Christians especially to designate monotheists (as Muslim) who do not subscribe to the Judeo-Christian concept of God and in such usage is distinguishable from heathen and pagan.
From the Muhammedan point of view, especially as presented in English fiction and poetry, infidel often means a Christian.