Of all the Egyptian religious beliefs that existed from the Prehistoric period down into Roman times, the oldest and the one most held in veneration was that connected with the worship of Osiris, Isis and Horus. These three, though primarily only local gods, at an early period became prominent deities of all Egypt; and the cult of Isis, more particularly, remained a favourite always, rivalling even that of Osiris in later times.
During the many thousand years of Egyptian history, not only did many changes occur in the ceremonies connected with these cults, but also the legends and origin of the Osirian faith received many additions and interpolations; and thus the old faith lost much of its purity. The simplest form in which it is preserved to us states that Osiris was the son of Seb and Nut—i.e. Earth and Heaven; of whom were born also Isis, Nephthys and Set, or Sutekh, as he is also called.
Osiris married his sister Isis, while Set chose Nephthys. It is probably due to this feature of the legend that the Pharaohs often married their sisters, and occasionally also their daughters. Osiris was the first divine ruler of Egypt: whence he came is not told; but when he came to that country, he found it sunk in barbarism and ignorance, with no law but that of strength, and poverty everywhere. He went through the land settling quarrels, organizing government, teaching polite manners and customs, dictating laws and civilizing the people.
Set, his brother, became jealous of the renown of Osiris, and hated him because of the good he had done; and resolved upon his destruction. Assisted by the evil spirits, or demons (the "enemies" or "foes" of the chants), Set constructed a large chest exactly the size and shape of the body of his brother Osiris; and at a feast given by the latter, he offered to present the chest, which was richly adorned, to the one whose body it best fitted. No one was successful until Osiris entered the chest, when Set closed it, and with the help of the evil spirits bore it from the banquet hall and cast it into the Nile. Thereupon Isis fled to the Delta, taking with her Horus, her son, whom she left to be cared for in Buto, according to some legends; others say he was left with Tehuti, or Thoth, god of learning. Isis then took a boat and searched the Nile for the body of her husband, which she ultimately found in the Delta. Before it could be interred, however, it was stolen by Set, who then divided the body into fourteen portions, and scattered them over the whole of Egypt. Again the bereaved Isis commenced a search for the pieces of her husband's body, and found thirteen; the fourteenth piece, the phallus, she was unable to find, it having been eaten by fish.
Wherever a fragment of the body of Osiris was found, a temple was there erected to his memory; and as the head (or according to some authorities, the heart) was found at Abydos, that city was considered as especially sacred, and was the centre of his worship.
Horus, son of Osiris and Isis, avenged his father's death by ultimately slaying Set; while Osiris, miraculously resurrected by Horus in the regions of the dead, ruled over the underworld and its inhabitants. Such, briefly, is the legend of Osiris. But the cult could never have become national in character without changing in many ways. Every city and town of any prominence in Egypt had its own especial local deity, who received special worship, even while other deities were admitted to exist, though considered subordinate to him: and at an early date the priesthood of Osiris began systematically to identify the leading characteristics of these various local gods with those of Osiris. Hence in course of time we find Osiris-Sokar, an identification of the local god Sokar with the great deity of Abydos; Osiris-Apis, which in Greek times became Serapis, where the attributes of Osiris had become identified with those of a Memphite deity; and in later times, Osiris became a solar deity, and is addressed by epithets and titles which seem to show an identification with Ra, the sun god. By this means, Abydos became the great early religious centre of Egypt. But these identifications of various deities were not confined to Osiris. Horus became mutated, and gained new attributes—as Horus-Ra, he became the midday sun; and under the priesthood of Heliopolis, he became Tum, the setting sun, even losing his name. Isis, also, received new qualifications, being often identified with Hathor (whom the Greeks in turn identified with Aphrodite); but the chief places where Hathor was worshipped were at Dendereh and at Der-el-Bahri, in the western portion of Thebes; while the worship of Isis centred around Abydos and Busiris, the latter being frequently mentioned in the liturgies; and in later times also she was worshipped at Philae; and her worship was so popular, that long after Egypt had ceased to be a nation, and her gods had become a by-word, there were no less than three great temples erected to her cult within the walls of Rome itself.