Dear Doubting Steve,
I'm glad you agree with me that the longevity of a belief, although it doesn't prove anything as such, does suggest it has some validity. Your examples, about an instinctive aversion to snakes or an instinct that serves to promote the species, were excellent ones. I wish I'd thought of them. Who could deny that those instincts are without some validity?
I'm also glad you agree with my argument that empiricists--and believers--have, at some point, to ask what lies behind the first cause. The difference, of course, is that while believers at times also suffer existential doubt at what exists behind God, they are consoled by the notion that God himself is the ultimate answer to that dilemma, and, in some ways, is defined as the answer to that dilemma. Empiricists have no answer at all.
Now, to Christianity. You are, I think, misrepresenting the core beliefs of Christians. You seem to argue that Christianity was a way to get around this existential dilemma by proving the existence of God by incarnating him in a human being. And you point out the silliness of this concept, because, of course, one simple human being, existing at a particular moment of time, with access to very few actual human beings, could hardly serve to prove God's existence to all mankind for ever and ever. And in this, of course, you are exactly right. That was never Christ's purpose; nor would it ever make sense.
The incarnation was not designed to prove anything empirically about the existence of God. It was designed to prove something about the nature of God. Because God exists, by definition, in a manner that is always going to be inaccessible to us--we are mortal, he is immortal; we are bound by the limits of human intelligence and experience; he is boundless and omnipotent--he could never be proven in the context of our limited, human, empirical world. What the incarnation was about, therefore, was not to show that God exists, but to show something far more implausible--that he is not indifferent to us. That, of course, is an enormously difficult proposition to defend, but Christ attempted to do so by his teachings and by his example. (You might recall that most of his miracles were not magic tricks; they were ways in which he touched people, helped them, reached out to them. They were about love, not omnipotence. And he was ambivalent about them, not wanting them to be used crudely as evidence of his divinity, but as evidence of his concern.)
And in any case, Christ's incarnation is not the ultimate indicator--even for Christians--of his divinity. The ultimate indicator of Christ's divinity is his resurrection. And even then, Jesus rose from the dead with no direct witnesses of the event. Even then, he pointed to faith, not the senses, as the appropriate way to understand the ultimate mysteries of life. Even when he appeared to the disciples after his resurrection, even when he asked doubting Thomas to place his hands in his still-gaping wounds, Christ was intent on saying that. "Blessed are those who have not seen and yet believe."
Until you appreciate this essentially anti-empirical core of Christianity, you will continue to ask Christians to prove what we do not think should be proven. Even for those first disciples, who saw and touched Christ himself, it was never a matter of evidence. It was always a matter of faith.