I think it might even help Kerry a little that he's Catholic, because a small number of Catholic voters might be swayed by it. I'm not that up on the demographics of Catholicism but I'm guessing it might help in PA and OH. For the most part though any Dem is going to win the heavily Catholic states, and there is no way any Dem is going to win any of the Protestant Bible Belt states, whatever the candidate's religion.
Western PA is heavily Catholic and is usually a swing vote. This would probably ensure a Dem victory in PA, but Dems would likely win it in a 50-50 race anyway.