1670s, "belief in a deity or deities," (as opposed to atheism); by 1711 as "belief in one god" (as opposed to polytheism); by 1714 as "belief in the existence of God as creator and ruler of the universe" (as opposed to deism), the usual modern sense; see theist + -ism.
Theism assumes a living relation of God to his creatures, but does not define it. It differs from deism in that the latter is negative and involves a denial of revelation, while the former is affirmative, and underlies Christianity. One may be a theist and not be a Christian, but he cannot be a Christian and not be a theist. [Century Dictionary]