A theologian (thee-oh-LOW-gin) is a person who studies religion. The word comes from the Greek word for god “theos” and the Greek word for writing “logos”. So a theologian is someone who writes about God. Usually when people say “theologian” they mean a Christian theologian.
For example, Montanus, Tertullian, Augustine, Gregory, Abelard, Heloise, and Thomas Aquinas were Christian theologians. But Jewish thinkers like Maimonides and Rashi were also theologians. So were Islamic thinkers like al Ghazali and Ibn Rushd.