Theism is broadly defined as the belief in the existence of at least one deity. In common parlance, or when contrasted with deism, the term often describes the classical conception of God that is found in monotheism (also referred to as classical theism) — or gods found in polytheistic religions — a belief in God or in gods without the rejection of revelation as is characteristic of deism. Gnosticism is the belief in personal spiritual knowledge.

Atheism is commonly understood as non-acceptance or rejection of theism in the broadest sense of theism, i.e. non-acceptance or rejection of belief in God or gods. Related, but separate, is the claim that the existence of any deity is unknown or unknowable: agnosticism. Combined with theism, is agnostic theism.