When Ptolemy I Soter took power, he created a new Egyptian god, Serapis. Serapis was a combination of Apis and Osiris, plus the Greek gods Zeus, Hades, Asklepios, Dionysios, and Helios. Serapis has powers over fertility, the sun, corn, funerary rites, and medicine. He is powerful, and is, in fact, the ancestor of Ptolemy... according to Ptolemy II. Yes, Ptolemy II has proclaimed his lineage in fact begins with the god Serapis, and that therefore not only is Ptolemy divine through his title, but divine through his dynasty. Only Ptolemies are allowed to rule Egypt, only Ptolemies are truly holy enough, only Ptolemies are true gods. Serapis was there when Alexander died, and that is why Alexander's body is held in the Temple of Serapis in Alexandria. Serapis has made the Nile flood, the sun shine, the air move, and waters flow, he is why we live, and it is the duty of the Egyptian people to worship him. Greek immigrants worship him. Greek natives worship him. Egyptians worship him.

And, as a result, they worship Ptolemy II.