This is an point of contention that seems, to me at least, to be easily understood and accounted for. Judaism did not blossom and mature in a vacuum; God spoke to, and interacted with, man over many centuries. Does anyone here actually think that the early Hebrews had no impact on their surrounding communities? Would it not make sense that as God revealed Himself to Abraham, Isaac and Jacob, their lives would have been ample demonstration of the power and glory of God? Would not this knowledge and understanding of God have leached into surrounding communities, affecting and attaching to undeveloped spiritual beliefs? And wouldn’t this necessarily limited knowledge have certainly resulted in change to that community?
Let’s take a simple story. Adam and Eve certainly knew the correct unfolding of events in the garden of Eden. Their kids would also have heard the story. The grandkids would have heard, and so on, generation after generation. At some point, gradually, the story changed: times expanded or contracted, blame was assigned, descriptions became less clear, etc. Now, over time, as families grew and separated, this ongoing deterioration of the truth continued. At some point it’s reasonable to presume that the main points would be lost to the fog and vagueries of memory and personal emphasis. In time, individual groups decide to write and record the events as they are now known, and voila you have a variety of storylines for one event. Then, God decides to correct the story, bring back the original acts to man’s awareness, and we have the book of Genesis. Great. Now, today, 3500+ years later, we have people looking for earliest references and assuming, yes assuming that the earliest records are the correct records. Of course, we then have to take into account the vagueries of archaelogical acquisition, don’t we? What has survived the ages and the destructive effects of the elements? What has survived war, natural disaster, intentional destruction by other means?
This is why I find this question to be of little intrinsic value. It presupposes that Christianity (rather Judaism, as the original communication with God) is some hybrid of earlier, less successful so-called religions, rather than a complete, complex, mature and unique theology, intended by God to counter the erroneous history of His relationship and purpose with His creation.