The introduction of foreign cults into Greece had always been easy. Eastern deities, such as Adonis, had already been introduced in Classical times. The most representative centers of the congregation of all manner of cults were the large ports and the cities founded during the Hellenistic and Roman periods. For instance, in Thessalonike the gods of the East, particularly of Egypt (Serapis, Isis, Harpokrates), already had many devotees in the Hellenistic years.
In those days there was a tendency to identify these eastern divinities with Greek ones (e.g. Serapis with Pluto, Isis with Demeter), or lent them their properties (e.g. Isis Lochia from Artemis Lochia, the protector of women giving birth). At the same time older minor Greek deities were promoted (e.g. Asklepios to Zeus Asklepios). The ceremonial of worship of these gods borrowed elements from the more elaborate eastern tradition.
This multiple identification of gods and a propensity to unify the worship, which intensified in the pre-Christian Roman years, led to a monotheistic trend, manifested either by attributing all properties to a single pagan god (Serapis Pantheos, the All Gods), or accepting the new monotheistic religion of Christianity.