Alongside his Egyptian roots he gained attributes from other deities, such as chthonic powers linked to the Greek Hades and Demeter, and benevolence derived from associations with Dionysus.

The Greeks had little respect for animal-headed figures, and so a Greek-style anthropomorphic statue was chosen as the idol, and proclaimed as the equivalent of the highly popular Apis.

The earliest mention of a "Sarapis" occurs in the disputed death scene of Alexander (323 BCE),[6] but it is something of a mixup: The unconnected Babylonian god Ea (Enki) was titled Šar Apsi, meaning "king of the Apsu" or "the watery deep",[b] and Ea as Šar Apsi seems to be the deity intended in the description of Alexander's death.

His significance to the Hellenic psyche, due to the mention in the story of Alexander's death, may have also contributed to the choice of the similar-sounding Osiris-Apis as the chief Ptolemaic god, even if the Ptolemies understood that they were different deities.

Ptolemy's project was to find a deity that would win the reverence of both groups alike, despite the curses the Egyptian priests had chanted against the gods of the previous foreign rulers (e.g. Set, who was lauded by the Hyksos).

Also, according to Tacitus, Serapis (i.e., Apis explicitly identified as Osiris in full) had been the god of the village of Rhakotis before it expanded into the great capital of Alexandria.

In his 2nd-century CE Description of Greece, Pausanias notes two Serapeia on the slopes of Acrocorinth above the rebuilt Roman city of Corinth, and one at Copae in Boeotia.

The Roman cults of Isis and Serapis gained in popularity late in the 1st century when Vespasian experienced events he attributed to their miraculous agency while he was in Alexandria, where he stayed before returning to Rome as emperor in 70 CE.