Sigmund Freud was also a student of religion as well as psychoanalysis. In his last book, Moses and Monotheism, 1939, he discusses the origin of Christianity. I will give my summary of the main ideas when I read the book.
He suggests here that the first Moses was an Egyptian and lived around the 13 or 14 century B.C. The Egyptian pharaoh at the time was Amenhotep (IV) who rejected the polytheism of the previous pharaohs and adopted monotheism during his reign of 17 years. He changed his name to Ikhraton, left Thebes and built a new capital further down the Nile (p. 38–39) which he called Akhetaton. Nefertiti was his wife. It was during this period that Moses lived and he embraced this new religion with Aten his God. Not wanting to abandon this new religion and revert to polytheism upon the pharaoh’s death, Moses and his Egyptian followers (Levites) selected the repressed Semitic tribes of Egypt to leave with him and this departure became the Exodus (p. 47). These various tribes with their new Egyptian religion crossed the Sinai and settled in the Hejaz. Over some hundreds of years and under a new Moses they lost their Egyptian religion and gained the polytheistic religion of the Arabic Midianites (p. 55) of which one of the Gods, the volcano god, was called Jahve (origin of John, Jahweh, Jehovah and so on). Jahve recognised the existence of other Gods and was jealous of them. But over time the repressed monotheism of the lost Egyptian religion came to the fore and reasserted itself and so Jahve became the only God with all other Gods now false Gods. Thus the Jewish religion (and so later Christianity) has as its origin both Egyptian and Arab components.
But is this enough? What was in Ikhraton’s mind when he decided to move away from polytheism and choose monotheism. Was is a genuine inspiration or did he know of earlier religions that were monotheistic? Are most things lost in time, like our knowledge of our ancestors?
“Moses and Monotheism” can be downloaded from Archive.org as it is out of copyright (greater than 70 years old).