It's relevant here to say that even those who believed in Jesus in early Christianity didn't all believe the same thing. I would have expected them to be closer to being in the know, since Jesus' death was closer in time to them than to us.
Let's see what Ehrman says in Lost Christianities, the Battles for Scripture and the Faiths We Never Knew:
"The wide diversity of early Christianity may be seen above all in the theological beliefs embraced by people who understood themselves to be followers of Jesus. In the second and third centuries there were, or course, Christians who believed in one God. But there were others who insisted that there were two. Some said that there were thirty. Others claimed there were 365.
In the second and third centuries there were Christians who believed that God had created the world. But others believed that this world had been created by a subordinate, ignorant divinity (why else would the wolrld be filled with such misery and hardship?). Yet other Christians thought it was worse than that, that this world was a cosmic mistake created by a malevolent divinity as a place of imprisonment, to trap humans and subject them to pain and suffering.
In the second and third centuries there were Christians who believed that the Jewish Scripture (the Christian "Old Testament") was inspired by the one true God. Others believed it was inspired by the God of the Jews, who was not the one true God. Others believed it was inspired by an evil deity. Others believed it was not inspired.
In the second and third centuries there were Christians who believed that Jesus was both divine and human, God and man. There were other Christians who argued that he was completely divine and not human at all (For them, divinity and humanity were incommesurable entities: God can no more be a man than a man can be a rock.) There were others who insisted that Jesus was a full flesh-and-blood human, adopted by God to be his son but not himself divine. There were yet other Christians who claimed that Jesus Christ was two things: a full flesh-and-blood human, Jesus, and a fully divine being, Christ, who had temporarily inhabited Jesus' body during his ministry and left him prior to his death, inspiring his teachings and miracles but avoiding the suffering in its aftermath.
In the second and third centuries there were Christians who believed that Jesus' death brought about the salvation of the world. There were other Christians who thought that Jesus' death had nothing to do with the salvation of the world. There were yet other Christians who said that Jesus never died.
How could some of these views even be considered Christian? Or to put the question differently, how could people who considered themselves Christians hold such views? Why did they not consult their Scriptures to see that there were not 365 gods, or that the true God had created the world, or that Jesus had died? Why didn't they just read the New Testament?
It is because there was no New Testament. To be sure, the books that were eventually collected into the New Testament had been written by the second century. But they had not yet been gathered into a widely recognized and authoritative canon of Scripture. And there were other books written as well, with equally impressive pedigress - other Gospels, Acts, Epistles, and Apocalypses claiming to be written by the earthly apostles of Jesus".
end of quote.
the terrible thing here is, can we believe that there were books written for the purpose of defending the views that the writer already held, but not necessarily for the purpose of telling the truth? YES. And the more orthodox defenders of the Canon would equally say that some of the books supposedly written by apostles were not written by them at all, as in the Acts of Peter, for example.
I believe that we do not have a way to know who wrote what and why. We don't know who changed what, when, or why.
