An alternative, unfalsifiable hypothesis is that someone existed who played a causal role in the founding of Christianity but that they were so boring they left no trace in history and hence bore very little resemblance to the figure described in the Bible.

Someone -- many people -- played a causal role in the founding of Christianity, since Christianity was, in fact, founded [1]. Does modern scholarship have anything to say about what did happen to start Christianity?

[1] I have just thought up an imaginary Christian heresy that claims that Christianity was in fact never founded at all. God planted it as an already well-developed sapling some time in the late 1st or early 2nd century "AD", giving everyone involved false memories of how it had started. That's why there's no historical evidence from Jesus' lifetime.