Theism is the belief that a god or gods exist. In its most common modern usage it refers to the conviction that there is one God who is personal, transcendent, and involved in the world—creating, sustaining, and in some traditions intervening.

Classical theism, shaped by Jewish, Christian, and Islamic thought as well as by Greek philosophy, ascribes to God attributes such as omnipotence, omniscience, perfect goodness, and eternity. God is not a part of the universe but its source; the relation is one of dependence rather than parity. Variations exist. Deism accepts a creator but denies ongoing intervention. Pantheism identifies God with the universe itself. Panentheism holds that the world exists within God without exhausting the divine reality.

Arguments for theism have ranged from the cosmological and teleological proofs of medieval and early modern philosophy to contemporary versions that draw on fine-tuning or consciousness. Critics counter with the problem of evil, the success of naturalistic explanation, and the diversity of religious claims. The debate continues in both academic and popular forms.

For the theist, the existence of God is not an optional hypothesis but the fundamental fact that makes sense of existence, value, and meaning. Everything else is secondary.