The Great Hymn to the Aten is the fullest surviving statement of the religious programme of Akhenaten, and what makes it remarkable is what it leaves out.
Egyptian religion was crowded. The hymn addresses a single god, the Aten, the visible disc of the sun, described as creating and sustaining everything, lands, peoples, languages, skin colours, the child in the womb, the chick in the egg. It describes the world at night, when the Aten sets, as a place of darkness where lions come out and thieves work, and the morning as a general awakening. It is a very good poem, attentive to animals and to ordinary human activity in a way little Egyptian religious writing is.
What it omits is Osiris, the underworld, judgement of the dead, and the whole apparatus of the funerary religion that dominated Egyptian practice for two thousand years. The hymn's implicit theology places the king as the sole intermediary: the Aten is known to Akhenaten alone, and everyone else approaches through him.
The text is inscribed in the tomb of the courtier Ay at Amarna, one of several Amarna tombs carrying versions of varying length. The traditional ascription to Akhenaten personally is plausible given the programme's dependence on him and is not demonstrable.
Whether this counts as monotheism is argued. The Aten is not presented as the only god who exists so much as the only one who matters, and the king and queen are still divine figures within the scheme. Henotheism or monolatry are the terms usually offered instead.