The reader's question, reworded by me, runs approximately like this:
It seems to the reader that the prior probability of the resurrection is an exception to the law of total probability. The reader asserts that P(R|~G) = 0. He also correctly points out that, on the assumption that P(R|~G) = 0, we should calculate P(R) = P(G) x P(R|G). The problem, the reader claims, is that multiplying the prior probability of God's existence by the probability that the resurrection takes place given God's existence appears to produce a probabilistic error. The reader produces a modus ponens argument:
p1= If God doesn't exist, then the resurrection is impossible.(The reader takes this to be analytically true.)
p2= God doesn't exist.
c= Therefore, the resurrection is impossible.
If premise 1 is analytic, one must deny premise 2 to deny the conclusion. But, says the reader, premise 2 need only be more plausible than not to be assertable. That would seem to mean that if P(G)<50%, the probability of the resurrection is 0, which, however, is not what we would get if we calculated the prior probability of the resurrection as we should using the law of total probability--that is P(R) = P(G) x P(R|G).
I'm going to waive the question of whether it is analytically true that the resurrection is impossible if God doesn't exist, because that either is simply a definitional matter (e.g., if you define "the resurrection" as an act of God) or involves a near-zero probability of a naturalistic resurrection.
The error in the reader's reasoning arises from his putting the wrong kind of weight--specifically, probabilistic weight--on the claim that one is justified in asserting that God doesn't exist if the probability of God's existence is less than .5. Even supposing that we grant that, that has no weight whatsoever for calculating the prior probability of the resurrection. You cannot go from, "'God does not exist' can be asserted justifiably if it is more probable than not" to "We should do our calculations of the probability of other propositions based on treating the probability of God's existence as 0 whenever the probability of God's existence is less than .5."
In essence, the above argument is a completely confused attempt to combine deductive and probabilistic reasoning. There would be no probabilistic inconsistency if the atheist were to say, viewing the prior probabilities, that probably the resurrection could not happen, or something like that. But that would have to be carefully spelled out by adding the word "probably" after "therefore" in the conclusion. (Compare "If John [defined by some definite description] doesn't exist, it is impossible for John to speak to me. John [defined by that definite description] doesn't exist. Therefore, it is impossible for John to speak to me.) The prior probability of R just is what it is. Nothing magical happens if the prior probability of G is below .5. Whatever the prior probability of G might be, you just plug that into the total probability calculation for the prior of R, and that's it. The modus ponens argument given simply doesn't tell us what the prior probability of R is.
Another way to put this is that you have already taken into account the assumption that the resurrection is impossible if God does not exist in the very act of reducing the prior probability of R to P(G) x P(R|G). Nothing more is required to take that assumption into account. The attempt to take it into account (somehow) more seriously by the modus ponens argument and the worry about what happens if the prior for G is less than .5 only darkens counsel.
The moral of the story: Don't mix apples and oranges, at least unless you're well-trained in the art of making apple-orange preserves. When you do probability, do probability. When you do deductive logic, do deductive logic. If you insist on mixing them, be verry, verry careful, or you could get yourself very, very confused.