His name means "the hidden one", and Egyptian theology took that literally rather than poetically. Amun is the god whose nature is concealment: the air you cannot see, the power behind appearances, the deity of whom the Leiden hymns say that his true form is unknown, that no god knows his real shape, and that all other gods are aspects of him while he remains apart. That is unusually abstract for Egyptian religion, and it was arrived at late, after his political rise had made him too important to be merely a local wind god.
From a minor Theban god to the richest institution in the world
He appears early — one of the eight primordial deities of the Hermopolitan Ogdoad is an Amun paired with Amaunet, representing hiddenness among the pre-creation conditions — but for the Old Kingdom he is not a major figure. His rise tracks the rise of Thebes.
Theban rulers of the Eleventh and then the Twelfth Dynasty carried his name: Amenemhat, "Amun is at the front". When Theban kings expelled the Hyksos and founded the New Kingdom in the sixteenth century BCE, they credited the victory to him, and they paid. Temple endowments in the New Kingdom transferred land, cattle, ships, mines, foreign tribute and personnel to Amun's estate on a scale documented in the Great Papyrus Harris, Ramesses III's inventory of donations. By the late New Kingdom the god's estate at Thebes was an economic power in its own right, with the High Priest of Amun a figure who could rival the crown — and at the end of the Twentieth Dynasty the high priests at Thebes ruled Upper Egypt in fact, while kings ruled from the Delta.
Karnak is the physical residue of that. It is not one temple but a precinct accreted over roughly two thousand years, with pylons, obelisks, a sacred lake and a hypostyle hall of one hundred and thirty-four columns whose central twelve rise about twenty-one metres. It is among the largest religious buildings ever constructed anywhere. Luxor temple, downstream, is joined to it by an avenue of sphinxes, and the annual Opet festival carried the god's barque between them in procession, with the king participating in a rite of royal renewal.
Amun-Ra
The fusion with Ra is the making of him. Ra was the ancient national solar god of Heliopolis with unimpeachable prestige; Amun was the ascendant god of the ruling city. Amun-Ra combined the hidden power with the visible sun and produced a national god who was both, styled king of the gods. Egyptian religion did this repeatedly and never deleted the originals — Ra continued to be worshipped as Ra — and the family tree draws such fusions as compound nodes as a merged node, since the combined god is neither god's child but both of them at once.
His Theban triad is Amun, Mut and Khonsu: father, mother and moon-child, each with a precinct at Karnak. His sacred animals are the ram, specifically the curved-horned variety whose recumbent form lines the Karnak approach, and the Nile goose.
Akhenaten, and the erasures
Around 1350 BCE Amenhotep IV changed his name to Akhenaten, moved the capital to a virgin site at Akhetaten — modern Amarna — and elevated the Aten, the visible sun disc, to sole official status. Amun's temples were closed, their revenues diverted, and agents were sent up and down the country to chisel out the god's name wherever it appeared, including inside the king's own father's cartouche. The damage is still visible at Karnak: the hacked-out patches are the physical trace of a state campaign against a deity.
It did not survive him. Within a few years of his death the court returned to Thebes, Tutankhaten became Tutankhamun, the temples reopened, and the Restoration Stela recorded the country's condition under the heresy in the bleakest terms. Later king lists omit Akhenaten entirely, and his city was quarried for stone.
Whether Atenism was genuine monotheism is one of the largest live arguments in Egyptology and belongs on the Aten page; what is not in dispute is that its principal target was this god and his priesthood.
Siwa, and Alexander
A remote oracle of Amun operated at the Siwa Oasis, deep in the Western Desert. Greeks knew it as the oracle of Ammon and treated it as authoritative — Herodotus reports it, and the Athenians consulted it. In 331 BCE Alexander the Great made a difficult desert journey to Siwa and emerged having been greeted, by the priest's account or his own, as son of the god. What was said is not recorded reliably; Alexander did not disclose it. Thereafter he was represented on coinage with the curling ram's horns of Amun, and those horns are why the Qur'an's Dhu al-Qarnayn, "the two-horned one", has been associated by many commentators with Alexander. A Theban god's animal ended up on the currency of a Macedonian empire.
The Greeks equated Amun with Zeus, giving Zeus Ammon; the Romans continued it as Jupiter Ammon.
Questions the hidden god attracts
Is the "amen" at the end of Christian prayers derived from Amun?
No. It is a Hebrew word meaning something like "truly" or "so be it", from a root concerning firmness and reliability, and it reaches Greek and Latin from Hebrew liturgy. The resemblance to the god's name is coincidental, and the claim recurs constantly online without ever acquiring evidence.
If he is "hidden", how was he depicted?
Usually as a man in a flat-topped crown carrying two tall plumes, with blue skin in many New Kingdom paintings; also as a ram, or a ram-headed man, and in the ithyphallic form Amun-Min, borrowed from Min of Coptos, marking generative power. The hiddenness is a theological statement about his essence, not a prohibition on images. Egypt saw no contradiction: the statue is a form the god can enter, not a portrait of what he is.
What did the Report of Wenamun say about him?
It is an Egyptian literary account, set at the close of the New Kingdom, of an envoy sent to Byblos to buy timber for the god's ceremonial barque, who is robbed, stranded, and treated with open contempt by a Levantine prince who is unimpressed by his credentials. Whether it is a report or a fiction, its interest is the same — it shows Egyptian prestige abroad collapsing, told from the perspective of a man carrying the authority of the country's greatest god and finding that it no longer buys anything.