I always come back to the doctrine of the Virgin Birth. IMO, on this doctrine the whole truth or lack thereof of Christianity hangs.
This is so on-its-face impossible for me to believe, and so beyond any attempts at literal proof, that I don't feel any obligation to PROVE that it didn't happen. This is one of the most extraordinary claims made about Jesus and I can't imagine how any extraordinary, or even ordinary proof could be provided authenticating it. It is a 100% leap of faith to believe it, and I sometimes wonder just how much hard thought this doctrine is given by persons professing Christianity, in terms of, 'this is just a willing belief in something entirely unprovable and therefore it is perfectly reasonable to reject it and therefore I cannot fault a soul for not believing it'.
Of course there are Christians who insist that "spiritual truths" don't necessarily have to be "literal truths" or whatnot, and I can respect that, I guess. But all it amounts to is a kind of existentialist Christianity, in other words, just choosing to believe something not because of its literal truth but because of its ability to provide meaning. I've tried that course but in the end it just doesn't work for me. Maybe I'll keep trying, I'm always a bit enticed by the Kiirkegaard school but there's just not enough meat on that bone to really chart your life course based on it.