During the Hellenistic period the coexistence of native people and Greek populations in the states that were created by the Diadochi of Alexander the Great, favoured the gradual spread of the eastern deities' cults: the Egyptian Isis and Sarapis, the Mother of the Gods, Cybele, and her lover Attis, the Phrygian Men, the Assyrian Atargatis, Hadad, Melkart, Astarte, Sabazius and many more. Initially, these deities became known in cosmopolitan cities, such as Rhodes, Delos, Corinth even in Demetriada in Thessaly. They became popular because they were equated or even identified with other Greek deities, especially the Olympians.

The most characteristic form of this phenomenon, which is called "religious syncretism", was the iconographic and verbal equation of deities who, in two or more devotional systems, had similar characteristics, bore the same symbols, were worshipped alike or had similar honours and sacrifices attributed to them. A prayer that was addressed to Isis by an Egyptian priest named Isidorus, in the 1st century BC, confirms syncretism: "The Syrians call you Astarte, Artemis, Anaia, the tribes of Lycians queen Leto, the Thracians Mother of the Gods, the Greeks glorious Hera, Aphrodite, good Hestia, Rhea and Demeter, but the Egyptians call you Thioue, because only you are every goddess the other people name".

In certain cases, this equation led to the integration of indigenous and Greek deities and to the creation of others, hybrid ones. Typical examples are the unifications of the Egyptian Thoth or Anubis with the Greek Hermes, from whom Hermes Trismegistus and Hermanubis respectively derived, as well as of the Egyptian Sarapis with Helios, from whom Heliosarapis derived.