To put it somewhat crudely, Paul claimed to have seen a ghost named Christ. He did not know Jesus the man who became Christ the ghost and he wasn’t interested in anything Jesus the man did before he died. He doesn’t say when or where Jesus the man lived or died. For Paul, what matters is what Christ the ghost did, not what Jesus the man did. Paul tells us that other people saw Christ the ghost, too, but not that they were companions of Jesus the man. Paul doesn’t claim to know anything about Jesus the man that Christ the ghost didn’t tell him.
Without some record of what Paul believed about Jesus the man, I don’t think that we can expect to have a record of someone challenging what Paul believed about Jesus the man. All anyone knew from Paul is that Jesus the man had been crucified for unspecified reasons at some unspecified time and place.
I can easily imagine that no one in Mark’s church would have listened to the idea that Jesus the man didn’t exist, but might not some of them have recognized his composition as a creative reconstruction based on Old Testament texts of what Jesus the man might have done rather than a record of stories passed down from eyewitnesses? We cannot establish external references to the gospels until well into the second century so there is at least the possibility that it took some time for Mark’s stories to gain widespread circulation and acceptance.
So the question becomes whether the origin of Christianity might lay in the visions that Paul and others had of Christ the ghost rather than in the activities of Jesus the man. Is it possible that the stories about the sayings and doings of Jesus the man were later additions? Personally, I don’t think that the evidence we have is sufficient to eliminate that possibility although I agree that much more work would be needed to establish that as the most likely scenario. That’s why I am agnostic on the question.
As Paul is our earliest source by some ten to twenty years, I don’t know how we could go about establishing that his correspondents had any more interest in Jesus the Man than Paul did. Our best and only direct evidence of what they knew about Christ the Ghost and Jesus the Man is what we find in Paul’s letters. It is surely not everything they knew, but I think that it may be everything that we can establish that they knew.
My hypothesis would be that as soon as people heard about Christ the Ghost who was the Messiah from Paul or others, they would have been curious about Jesus the Man. If in fact, no one claimed to have known Jesus the Man personally (which seems to me to be a possibility that we can’t eliminate), the logical place to look for information about him would have been in the Old Testament prophecies that the Messiah would have had to fulfill.
One possible hypothesis would be that Paul’s preaching was based entirely on his encounter with Christ the Ghost, but as his converts spread the message to others, it proved effective to include stories about Jesus the Man. The more these stories proved to be effective, the more of them would have been invented or “discovered” in the Old Testament.
I think it is a legitimate question whether there was enough time for this to occur, but I think the potential time frame is a bit longer than twenty years, which is the time from when Jesus the Man was crucified until Paul’s writings. I would add to that another twenty or so years until stories about Jesus the man start being written and perhaps as much as another fifty years before we have solid evidence that the stories were generally accepted.
Again, I don’t claim that we can know that this is what happened. But it seems to me that the evidence allows it as a logical possibility that cannot be fully eliminated.