I'm sorry, but this is coming into a dangerous ground. Many personal facts (including, but not only, private religious convictions) are simply not relevant to someone's performance as a politician, as evident in many examples of past and present leaders. And I don't like an idea of people's votes being determined by such reasoning as "hmm, he's a Catholic (or whatever else), so it may theoretically affect his policy". I know it's not what you said, but many other people would be inclined to follow this path.
You might very well be right on how we should evaluate a politician. On the flip side a politician has put himself or herself in the public domain and his or her religious beliefs are part of profile to be scrutinized by the public. One can say all day that the general public should not not certain parts of the profile in making their judgement but it is reasonable to ask about it and it is reasonable for the politician to reuse to answer and the voting public draw their own conclusions about that behavior.