1660s, from Greek theos "god" (see Thea) + -ist. The original senses was that later reserved to deist: "one who believes in a transcendant god but denies revelation." Later in 18c. theist was contrasted with deist, as allowing the possibility of revelation.
"belief in a deity," 1670s; see theist. Meaning "belief in one god" (as opposed to polytheism) is recorded from 1711.