There's a big difference between "no matter what" and "if He appeared to you himself," especially if by the latter you mean appearing to my senses. I mean, the immediate anecdotal evidence of my senses is far from being the most convincing form of evidence in my world; there are many things I'm confident exist without having directly perceived them, and some things I've directly perceived I'm confident don't exist.

For example, a being possessing the powers attributed to YHWH in the Old Testament, or to Jesus in the New Testament, could simply grant me faith directly -- that is, directly raising my confidence in that being's existence. If YHWH or Jesus (or some other powerful entity) appeared to me that way, I would believe in them.

I'm assuming you're not counting that as convincing me, though I'm not sure why not.

But if there is absolutely nothing that would convince you, then you're giving it a chance of 0.

Actually, that isn't true. It might well be that I assign a positive probability to X, but that I still can't rationally reach a state of >50% confidence in X, because the kind of evidence that would motivate such a confidence-shift simply isn't available to me. I am a limited mortal being with bounded cognition, not all truths are available to me just because they're true.

But it may be that with respect to the specific belief you're asking about, the situation isn't even that bad. I don't know, because I'm not really sure what specific belief you're asking about. What is it, exactly, that you want to know how to convince me of?

That is... are you asking what would convince me in the existence of YHWH, Creator of the Universe, the God of my fathers and my forefathers, who lifted them up from bondage in Egypt with a mighty hand an an outstretched arm, and through his prophet Moses led them to Sinai where he bequeathed to them his Law?

Or what would convince me of the existence of Jesus Christ, the only begotten Son of God, who was born a man and died for our sins, that those who believe in Him would not die but have eternal life?

Or what would convince me of the existence of Loki, son of the All-Father Odin who dwells in highest Asgard, and will one day bring about Ragnarok and the death of the Gods?

Or... well, what, exactly?

With respect to those in particular, I can't think of any experience off-hand which would raise my confidence in any of them high enough to be worth considering (EDIT: that's hyperbole; I really mean "to convince me"; see below), though that's not to say that such experiences don't exist or aren't possible... I just don't know what they are.

With respect to other things, I might be able to.