The evidence that Jesus ever existed is weaker than you might think

[Major Nikon]

54. Judeo-Christian myths are collections of oral history

Eventually this oral history found it's way to written form generations and in some instances centuries after the events they describe. As such it's a safe bet many of them are going to be adapted from earlier oral histories.

All of this happened during a period in which small bands of nomadic tribes were coalescing into larger civilizations by virtue of conquest and cross-cultural trade. When one culture is assimilated or associates with another such things will blend together. Monotheism and Christianity's tritheism simply worked more efficiently as a method of control which is why they were mandated throughout western and middle eastern civilizations. But it's not as if paganism was going to simply vanish. It was instead absorbed and modified to make it more palatable.