thh Egyptians had many gods. One pharaoh worshipped a single god, the sun god Aten, around 1350 BC; after his death the old gods were restored. The single Israelite god dates back much further - to Abraham perhaps, about 2000BC.
The OT is a sort of ideological history of the wanderings of the Israelites and the prophets among them who relayed God's word to them. This OT God is not far from the Jewish and Muslim gods.
The basis of Christianity is that he changed his mind and sent his son to live among men; without actually rejecting any of the words of his father, Jesus modified them, insisting that love, not war, is the divine message. Under this 'new covenant', God had changed. This NT God is the God most Christians worship, and is distinct from the Jewish and Muslim gods - they don't accept that Jesus was the messiah. The fire-breather of the OT is mostly of historical interest.
And to reiterate: the gospels seem to have been written down 50-100 years after Jesus's death but that doesn't mean that they hadn't originally been handed down, in written or oral form, from eyewitness accounts. I'm not saying they were, just that we don't know. The fact that they disagree on details doesn't mean they're all wrong.