Suppose I agree with theists that "God exists" is a necessary proposition, and so is either a tautology or contradiction. That seems to indicate that the probability of "God exists" is either 1 or 0. Suppose also that I don't know which it is, but I find the evidential argument from evil convincing, and so rate the probability of "God exists" at, say, 0.2. But if the probability of "God exists" is either 1 or 0, then it can't be 0.2 - that would be like saying that "God exists" is a contingent proposition, which I've accepted it isn't. How then can I apply probabilistic reasoning to "God exists" at all? If I can, then how should I explain the apparent conflict?

I confess I don't understand the notion of "metaphysical necessity," if it does not entail that that there is no possible world in which the "metaphysically necessary" being does not exist. But only a pencil exists in world W. So I really don't see what is gained (or why the very question of God's existence is not simply begged) by the claim that God is a (metaphysically) necessary being.
If "God exists" is necessary, then the probability that God exists is 1. Full stop. It is not either 1 or 0, it is simply 1. It is also not 0.2 or any other number. Nothing like begging the question big-time, eh? On the other hand, I can't see why anyone serious about the question of God's existence (even theists, who would like the answer to be affirmative, but presumably not on foolish grounds) would accept the claim that "God exists" is necessary. If that were true than the could be no possible world (=a world that can be described without contradiction) in which God did not exist. But it seems obvious that there can be such a world. Consider this description: World W = a world in which only a single pencil exists. It's hard to spot the contradiction in that simple world! It would be a pretty boring place to be...but wait! If anyone were to be there, it would be a different world! Whew!