Before anything, there is water — not an ocean but an undifferentiated, dark, motionless, boundless wetness called Nun, in which nothing is separate from anything else. Atum is what happens when something first distinguishes itself from that. His name is built on a root that means both "complete" and "not exist", and Egyptian theology exploited the double sense deliberately: he is the totality, and he is the one who came out of nothing. The Coffin Texts have him describe himself, before creation, as alone in the waters, inert, with no place to stand.
The mound, and the mechanism
He brings himself into being, and a mound of land rises under him — the benben, the first solid thing, the model for every primeval mound and obelisk tip in Egypt. Then he creates the first pair, and the Egyptian sources are unembarrassed about the method.
The Pyramid Texts, the oldest religious corpus in the world, state that he took his phallus in his fist and produced Shu and Tefnut by his own hand — with the hand itself receiving a theological identity, the Hand of Atum, personified as a goddess and later given the title God's Hand held by high priestesses at Thebes. An alternative in the same corpus has him spit or sneeze them out, and the texts pun on the names: Shu against the word for sneeze or spit, Tefnut against a word for spitting. Both versions circulated together, and no Egyptian source treats them as competing accounts requiring resolution.
Modern retellings soften this constantly, and the softening is a real loss. Egyptian creation is a physical event performed by a body, without a partner, and the choice of masturbation is not crude humour but a precise theological solution to the problem of how a single being produces a second one. There is nobody else to help.
The Ennead
From Shu (air) and Tefnut (moisture) come Geb (earth) and Nut (sky); from Geb and Nut come Osiris, Isis, Set and Nephthys. Nine gods in four generations, the Heliopolitan Ennead, and Atum at the head of it. The word Egyptians used, psedjet, simply means "the Nine", and it is a designed structure — the family tree sets out why its regularity is so unlike the rest of Egyptian genealogy.
Heliopolis, Egyptian Iunu and biblical On, lay at the apex of the Delta near modern Cairo. Almost nothing of it survives above ground; one obelisk of Senusret I still stands in the suburb of Matariya. The theology it produced outlasted the city so completely that it is still the framework in which Egyptian religion is taught.
Atum and Ra
The relationship between Atum and Ra is the standard example of Egyptian theological merging. As Heliopolis's solar cult consolidated, the two were combined into Ra-Atum without either being abolished, and a threefold scheme distributes the sun's day across three names: Khepri the scarab at dawn, Ra at noon, Atum at evening. On that scheme Atum is the setting sun — the completed sun, the day's totality, the one going down into the Duat.
Which came first is not recoverable and probably not a well-formed question; both are attested from very early. What is clear is that Atum keeps the abstract and philosophical end of the pair. Ra sails, fights and grows old. Atum is the one the Coffin Texts have declare that at the end of things he will return everything to the waters and be left alone with Osiris in the form of a serpent, all creation undone. That is one of the very few eschatological passages in Egyptian literature, and it is startlingly bleak — creation is not permanent, only long.
How he is shown
Uniquely among the great creator gods, he is normally depicted as a plain human being, an old man, wearing the double crown of Upper and Lower Egypt. The crown is the tell: the first god wears the royal headgear because kingship is understood as extending directly from him, and the king's mythic ancestry runs up this line. A common temple scene has Atum, with Thoth or Horus, pouring water over the king in the purification rite before coronation.
His animals are more surprising. The ichneumon, the Egyptian mongoose — a snake-killer, and therefore an enemy of the disorder Nun contains — appears in bronze votive figures with the god's name. The eel and the lizard are also his, as is the serpent, the form he takes at the end of the world. Each carries the same logic: creatures that emerge from mud, water and holes, from the undifferentiated into the visible.
Questions about the first god
If Atum made everything, why do other cities say someone else did?
Because there was no central religious authority in Egypt and no scriptural canon, and each major temple's theology asserted the primacy of its own god. Memphis said Ptah conceived the world in his heart and spoke it — and the Shabaka Stone explicitly subordinates Atum's bodily creation to Ptah's word. Hermopolis said the eight primordial deities of the Ogdoad preceded everything. Thebes said Amun was hidden before all of them and the others were his manifestations. Esna said Neith did it. These systems ran in parallel for millennia, and Egypt combined them rather than adjudicating between them. A page about Egyptian creation that gives one account without saying which city it belongs to is describing a local claim as though it were a national one.
Was he actually worshipped, or just a theological principle?
Both, but the balance tips towards principle. He had a real cult at Heliopolis and appears in temple scenes across the country, especially in coronation and purification contexts, and Late Period bronzes of his sacred animals were dedicated in quantity. But he generates no myths of his own after the creation, has no festival calendar comparable to Hathor's or Amun's, and does not intervene in anything. He is where the system starts, and systems tend not to pray to their axioms.
What is the "Hand of Atum" doing as a priestly title?
The god's hand, as the instrument of creation, was personified as a female counterpart — sometimes identified with Hathor, sometimes with a goddess named Iusaaset or Nebethetepet. From the Third Intermediate Period the title God's Hand was held by the God's Wife of Amun at Thebes, an office occupied by royal women who wielded very substantial power in Upper Egypt. A theological detail about the origin of the universe became, a thousand years later, a job for a princess with an estate and a staff.