The Heliopolitan Theological System declares that in the beginning there was nothing but immense chaos, Nun, thought to be an ocean or shapeless magma containing, none the less potential life.
In this chaos there had been a conscious principle in existence since antiquity began, the god Atum, whose name means “the Whole,” “the Complete,” emphasized his abstract and somewhat metaphysical nature.
This god was the first of a divine line with each ensuring generation representing one aspect or element of the universe.
Su was the personification of air, in the sense of the void, and also in the sense of a life giving substance, whereas his sister/consort, Tefnut, whose role was less clearly defined, represented the moisture in the atmosphere.
From this first couple came Geb, the earth god, and Nut, the sky goddess, making the second couple. Shu was depicted standing in between the two raising and holding the body of the sky-goddess above the earth god.
From the couple came the other four divinities: Osiris and Isis, Seth and Nephthys. This Heliopolitan System continued until there were eight gods, nine including Atum, ruling the universe. Atum eventually became associated with Re, the major Egyptian sun-god.
This theological system which began in the Delta, at Heliopolis not far from the head of the Delta, was very well accepted because Atum could easily be made chief god in a local area, or a local goddess could become his consort, which eventually increased the Egyptian pantheon.
The acceptance of the sun-god, Re/Atum, greatly fitted the mindset of the ancient Egyptians living along the Nile.
The sun was seen as the barque crossing the ocean of the sky in the daytime, and the night boat which carried from one critical point to the other to begin his journey across the sky again.
In this way the people thought the sun god’s journey was uninterrupted. A.G.H.
Grimal, Pierre, Larousse World Mythology, Secaucus, New Jersey, Chartwell Books, 1965, pp. 30-32