When you believe in more the one god; The act of believing in many gods; The antonym for Monotheistic.
Polytheistic: The Greeks believed in more than one god. In fact, they believed in over 20 gods. A god of the Sea (Poseidon), a god of war, wisdom and skill (Athena), a god of love (Eros), and a god of thunder (Zeus), who was the main god. There was also a god of the Underworld (Hades).