With the spread of the Osirian form of faith the doctrine of the Trinity became universal throughout Egypt. The organisation of the faith had included the reduction of the cycle of divinities connected with Osiris into a trinity. Thoth and Anubis, Nebhât and Set, were separated from him, and henceforth he was made the head of a triad, in which Isis was the second person, and Horus, the avenger of his father, was the third. How completely the father and son were merged together may be seen from a hymn to Horus which has been translated by Chabas(185)--
"The gods are joyous at the arrival of Osiris, the son of Horus, the intrepid, the truth-speaking, the son of Isis, the heir of Osiris. The divine chiefs join him, the gods recognise the omnipotent child himself ... the reign of justice belongs to him. Horus has found his justification, to him is given the title of his father; he appears with the _atef_-crown by order of Seb. He takes the royalty of the two worlds, the crown of Upper Egypt is placed upon his head. He judges the world as he likes, heaven and earth are beneath his eye, he commands mankind--the intellectual beings, the race of the Egyptians and the northern barbarians. The circuit of the solar disc is under his control; the winds, the waters, the wood of the plants, and all vegetables.... Sanctifying, beneficent is his name ... evil flies afar off, and the earth brings forth abundantly under her lord. Justice is confirmed by its lord, who chases away iniquity. Mild is thy heart, O (Osiris) Un-nefer, son of Isis; he has taken the crown of Upper Egypt; for him is acknowleged the authority of his father in the great dwelling of Seb; he is Ra when speaking, Thoth when writing; the divine chiefs are at rest."
Here Osiris is identified with Horus, and so becomes the son of his own wife.
The Egyptian trinity has thus grown out of the triad under the influence of the solar theology, and of the old conception of a personality which possessed a concrete form. Once introduced into the Osirian creed, it spread with it throughout Egypt, and became a distinguishing feature of Egyptian theology. Along with the doctrines of the resurrection of the body and of a judgment to come, it passed into the schools of Alexandria, and was there thrown into the crucible of Greek philosophy. The Platonic doctrine of ideas was adapted to the Egyptian doctrine of personality, and the three persons of the trinity became Unity, Mind, and Soul--absolute thought, absolute reason, and absolute energy.(186)