Such a person is a monotheist.

When Christianity became the official state religion, other religions began to decline.

polytheism

It was progressively swallowed up by Alexander the Great, and on his death was divided up between his generals into the Hellenistic Kingdoms and given a veneer of Greek culture.

The assault by Alexander the Great, who replaced the empire with his own Macedonian Empire. It was split up into several Hellenistic kingdoms after Alexander's death.