Aten, the god of Atenism, first appears in texts dating to the 12th dynasty, in the Story of Sinuhe. During the Middle Kingdom, Aten "as the sun disk...was merely one aspect of the sun god Re." It was a relatively obscure sun god; without the Atenist period, it would barely have figured in Egyptian history. Although there are indications that it was becoming slightly more important during the eighteenth dynasty, notably Amenhotep III's naming of his royal barge as Spirit of the Aten, it was Amenhotep IV who introduced the Atenist revolution in a series of steps culminating in the official installment of the Aten as Egypt's sole god. Although each line of kings prior to the reign of Akhenaten had previously adopted one deity as the royal patron and supreme state god, there had never been an attempt to exclude other deities, and the multitude of gods had always been tolerated and worshipped. During the reign of Thutmosis IV, it was identified as a distinct solar god, and his son Amenhotep III established and promoted a separate cult for the Aten. There is no evidence that Amenhotep III neglected the other gods or attempted to promote the Aten as an exclusive deity.
Amenhotep IV initially introduced Atenism in the fifth year of his reign (1348/1346 BC), raising Aten to the status of supreme god, after having initially permitted continued worship of the traditional gods. To emphasise the change, Aten's name was written in the cartouche form normally reserved for Pharaohs, an innovation of Atenism. The religious reformation appears to coincide with the proclamation of a Sed festival, a sort of royal jubilee intended to reinforce the Pharaoh's divine powers of kingship. Traditionally held in the thirtieth year of the Pharaoh's reign, it possibly was a festival in honour of Amenhotep III. Some Egyptologists think that he had a coregency with Amenhotep IV of 2-12 years.
The fifth year is believed to mark the beginning of Amenhotep IV's construction of a new capital, Akhetaten (Horizon of the Aten), at the site known today as Amarna. Evidence appears on three of the boundary stelae used to mark the boundaries of this new capital. Then, Amenhotep IV officially changed his name to Akhenaten (Spirit of the Aten) as evidence of his new worship. The date given for the event has been estimated to fall around January 2 of that year. In the seventh year of his reign (1346/1344 BC), the capital was moved from Thebes to Akhetaten, but construction of the city seems to have continued for two more years. In shifting his court from the traditional ceremonial centres, he was signalling a dramatic transformation in the focus of religious and political power.
The move separated the Pharaoh and his court from the influence of the priesthood and from the traditional centres of worship, but his decree had deeper religious significance too. Taken in conjunction with his name change, it is possible that the move to Amarna was also meant as a signal of Akhenaten's symbolic death and rebirth. It may also have coincided with the death of his father and the end of the coregency. In addition to constructing a new capital in honor of Aten, Akhenaten also oversaw the construction of some of the most massive temple complexes in ancient Egypt, including one at Karnak and one at Thebes, close to the old temple of Amun.
In the ninth year of his reign (1344/1342 BC), Akhenaten declared a more radical version of his new religion, declaring Aten not merely the supreme god of the Egyptian pantheon but the only God of Egypt, with himself as the sole intermediary between the Aten and the Egyptian people. Key features of Atenism included a ban on idols and other images of the Aten, with the exception of a rayed solar disc in which the rays (commonly depicted ending in hands) appear to represent the unseen spirit of Aten. Aten was addressed by Akhenaten in prayers, such as the Great Hymn to the Aten: "O Sole God beside whom there is none". Aten's name is also written differently after the ninth year of the Pharaoh's rule to emphasise the radicalism of the new regime. Aten, instead of being written with the symbol of a rayed solar disc, now became spelled phonetically.