I don't know if the idea of atheism existed in Akhenaten's time, however his religion was definitely a step in that direction. Atenism did involve worship of the sun disc as well as Akhenaten himself (and Amenhotep III before him). I agree that it approximates modern atheism better than modern monotheism.
The Aten faith, if it is proper to refer to it as such, probably was designed to capture the "true spirit" of Ra/Re, which seems to have been a technical and rational individual rather than mystical. Ra/Re (as embodied in Akhenaten/Moses) represented a move away from superstitious and ecstatic practices. Osiris religion (as embodied in Tutankhamun/Elisha) represented a move back toward the superstitious and ecstatic.
The ancients seems to recognize the need for reason and emotion, but not necessarily both at the same time. Emphasis of religion, as with all else, moved in cycles. For example, Baal worship was appropriate in some periods but to be suppressed in others. Unfortunately there wasn't always agreement within the royal family regarding "what time is it".
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