This stems from the fact that, so far, the earliest dated written fragments we have from what is now the New Testament are some of the writings of Paul.
Paul was not one of the Apostles EDIT: Disciples, and it seems possible that, after persecuting earlier, existing Christians, he could have basically had a stress induced psychotic break and hallucinated the vision of Jesus that he had, then converted.
Thing is though, Christians would have to ... you know exist and already be a real thing first, for that to make sense.
It does explain why Paul does not mention some very key elements of the narrative of the Gospels: He just had not actually read about or heard of those parts yet.
This creates some theological problems down the line, and some of those problems were 'remedied' by what a good deal of scholars and historians believe to be forgeries... chapters of the Bible that modern Christians attribute to Paul, but do not seem to actually have been written by Paul.
It is also possible to some of the empty tomb accounts in some of the Gospels as similar kinds of trauma induced hallucinations.
Mark famously originally just ends with an empty tomb, and nobody said anything about this because they were scared... and then the last bit of verses giving Mark a more satisfying ending have been shown to be added ... decades later.