According to Plutarch, Ptolemy Soter stole the iconic statue from Sinope, having been bidden by the unknown god in a dream to bring him to Alexandria. On its arrival the statue was pronounced to be Serapis by two experts in religious matters: the one the Eumolpid Timotheus, the other the Egyptian Manetho. This story may not be true (some contend that Sinope as the provenance of the statue originated in the hill of Sinopeion, i.e. place of Apis (?), a name given to the site of the Serapeum at Memphis), but there is little doubt that Ptolemy Soter fixed the iconic type to serve for the god of the new capital of Egypt, where it was soon associated with Isis and Harpocrates in a triad.
Ptolemy's policy was evidently to find a deity that should win the reverence alike of Greeks and Egyptians. The Greeks of that day would have had little respect for an animal-headed Egyptian figure, while the Egyptians were more willing to accept divinity in any shape. A Greek statue was therefore chosen as the idol, and it was proclaimed as the anthropomorphic equivalent of a much revered and highly popular Egyptian beast-divinity, the bull Apis, assimilated to Osiris, god of the underworld. The Greek figure probably had little effect on the native ideas, but it is likely that it served as a useful link between the two religions.
The patron god of Alexandria soon won an important place in the Greek world. The purely human figures of Isis and Horus were easily rendered in Greek style, and Anubis was prepared for by hellenic ideas of Cerberus. The worship of Serapis along with Isis, Horus and Anubis spread far and wide, reached Rome, and ultimately became one of the leading cults of the west. The destruction in 385 of the Serapeum of Alexandria, and of the famous idol within it, after the decree of Theodosius, marked the death agony of paganism throughout the empire.
It is assumed above that the name Serapis (so written in later Greek and in Latin, in earlier Greek Sarapis) is derived from the Egyptian Userhapi as it were Osiris-Apis the name of the bull Apis, dead and, like all the blessed dead, assimilated to Osiris, king of the underworld. There is no doubt that Serapis was before long identified with Userhapi; the identification appears clearly in a bilingual inscription of the time of Ptolemy Philopator (221 - 205 B C), and frequently later. It has, however, been contended that the parallel occurrence of the names Sarapis and Osorapis (Userhapi) points to an independent origin for the former. But doublets, e.g. Petisis-Petsis, are common in Hellenized Egyptian names. The more accurate form is then generally the later, found in documents written by Greeks in familiar intercourse with Egyptians, the less accurate is traditional from an older date in the mouths of pure Greeks and Hellenists, and is used in literary writings. Thus Sarapis would be the literary and official form of the name; it might be traditional, dating perhaps from the reign of Amasis or from the Persian period. We know that in Herodotus's day, and long before, the discovery of the new Apis was the occasion of universal rejoicing, and his death of universal mourning. The ancient Serapeum (Puserhapi) and the name Userhapi would be almost as familiar to early Greek wanderers in Egypt as the Apieum and Apis itself.
But why was a Plutonic Serapis selected rather than another god to furnish the Egyptian element to the chief divinity of Alexandria? According to one account in Tacitus, Sarapis was the god of the village of Rhacotis before it suddenly expanded into a great capital; but it is not very probable that temples were erected to the dead Apis except at his Memphite tomb. Alexander had courted Ammon. But Ammon had little hold on the affections of the Egyptian people. He was the god of Ethiopia and the Thebais which were antagonistic to the progressive north. On the other hand, Osiris with Isis and Horus was everywhere honored and popular, and while the artificer Ptah, the god of the great native capital of Egypt, made no appeal to the imagination, the Apis bull, an incarnation of Ptah, threw Ptah himself altogether into the shade in the popular estimation. The combination of Osiris and the Apis bull which was found in the dead Apis was thus a most politic choice in naming the new divinity, whose figure represented a god of the underworld wearing an emblem of fruitfulness.
The earliest mention of Sarapis is in the authentic death scene of Alexander, from the royal diaries (Arrian, Anabasis, VII. 26). Here Sarapis has a temple at Babylon and is of such importance that he alone is named as being consulted on behalf of the dying king. It would considerably alter our conception of the dead Apis if we were to find that a travelling shrine of his divinity accompanied Alexander on his expedition or was set up for him in Babylon. On the other hand, the principal god of Babylon was Zeus Belus (Baal Marduk), and it is difficult to see why he should have been called Sarapis on this occasion. Evidence has, however, been found to prove that Ea, entitled Sarapsi, king of the deep (sea), who was also great in learning and magic, had a temple in the city. It seems unwarranted to make this Sarapsi-Sarapis travel to Sinope and thence to Alexandria as the type of the Egyptian god; but whether or not the Egyptian appellation Sarapis was applied to express the Babylonian Sarapsi, the part it played in the last days of Alexander may have determined the choice by which the Egyptian Osiris-Apis supplied the name and some leading characteristics to the god of Alexandria.
Serapis was especially worshipped as a god of healing, and his temples were connected with dream-oracles that were much resorted to.