on the ball –
You can’t just jump to mesorah to solve all your problems. The fact of the matter is that if I say kabbalah is bunk I would be thrown out of many Batei Medrash. Why? There’s no “Kuzari Principle” for it! For that matter there’s no “Kuzari Principle” for anything other than the Aseres HaDibros really. Why am I mentioning the Kuzari Principle? Because that is exactly your argument. You said that “one guy showing up doing a few magic tricks doesn’t change an ancient belief system.” Well why not? Why don’t a few magic tricks prove anything? After all, a navi is believed after he does “a few magic tricks.” What’s the answer – that the fact that our belief system is ancient means that it can’t be challenged? What kind of argument is that? Unless that argument is based in some kind of Kuzari argument it just sounds like nonsense and sophistry to me. Hence my point.
The fact is, anyone with half a Gemara kup can figure out answers to any arguments you will pose to the missionaries. So in truth why don’t we all convert? Because we have our mesorah – i.e. if our brand of Judaism was good enough for the tanaim and amoraim then it’s good enough for us as well; and because they have never proven that we should convert. We can debate with them on the defensive, i.e. when they show you a pasuk demanding that you believe in Jesus you show them that it is not at all muchrach to read the pasuk that way. But I do not think that there is any point in going on the offensive and trying to “refute” their religion. And as and aside I do not think historically we ever have done so.