After the Greek conquest, the new city of Alexandria became the centre of Egyptian religious life, and indeed of the religious life of the whole Hellenic world. Eventually in Egypt the hellenic pharaohs decided to produce a deity that would be acceptable to both the local Egyptian population, and the influx of hellenic visitors, to bring the two groups together, rather than allow a source of rebellion to grow.
Thus Osiris was identified explicitly with Apis, really an aspect of Ptah, who had already been identified as Osiris by this point, and a syncretism of the two was created, known as Serapis, and depicted as a standard Greek god.
A great temple, the Serapeum, was set up by Ptolemy I at which a sort of trinity of gods was worshipped. There were Serapis (who was Osiris-Apis rechristened), Isis and Horus. There were not regarded as separate gods but as three aspects of one god, and Serapis was identified with the Greek Zeus, the Roman Jupiter and the Persian sun-god.
This worship spread wherever the Hellenic influence extended, even into North India and Western China. The idea of immortality, an immortality of compensations and consolation, was eagerly received by a world in which the common life was hopelessly wretched. Serapis was called