One who believes that there is but one God.

One who believes in the existence of a single, usually omnipotent Deity. Christianity posits one male God; some feminist Pagan traditions posit one female Goddess. In its less enlightened forms, this can lead to hostility towards other religions, but it does not have to.

a believer in one god

a person who tries to convert worship of deities to the worship of just one god or deity

Someone who is, in fact, an atheist toward all the gods people have ever invented, except one. Someone who is only one god away from being right.