"God punishes us for believing" has a much lower probability, because no one believes it, while many people believe in Christianity.
Why does the probability have anything to do with the number of people who believe it?
"Muslims are right" could easily be more probable, but then there is a new Wager for becoming Muslim.
There's then the problem that the expected value involves adding multiples of positive infinity (if you choose the right religion) to multiples of negative infinity (if you choose the wrong one), which gives you an undefined result.
The probabilities simply do not balance perfectly. That is basically impossible.
The probability of any kind of God existing is extremely low, and it's not clear we have any information on what kind of God would exist conditioned on some God existing.
There's also the problem that if you know the probability that God exists is very small, you can't believe, you can only believe in belief, which may not be enough for the wager.
The probability has something to do with the number of people who believe it because it is possible that some of those people have a good reason to believe it, which automatically gives it some probability (even if very small.) But for positions that no one believes, this probability is lacking.
That adding positive and negative infinity is undefined may be true mathematically, but you have to decide one way or another. And it is wishful thinking to say that it is just as good to choose the less probable way as the more probable way. For example, there are ...
It has been claimed on this site that the fundamental question of rationality is "What do you believe, and why do you believe it?".
A good question it is, but I claim there is another of equal importance. I ask you, Less Wrong...
What are you doing?
And why are you doing it?