As with any employee, if their religious beliefs are completely separate from their job performance, I don't care what they believe. This might be possible in a presidential candidate if the person doesn't really take their religion seriously (i.e., a Christmas and Easter only attending Christian). But, I think that is nearly impossible for the devoutly religious. For the devoutly religious, their whole world view is filtered through their religious beliefs. Their understanding of world events and how they should be solved is influenced by their religious beliefs.
The worldview of a presidential candidate is relevant. Their epistemology (how they learn truth) is relevant. Their views on social issues are relevant. All of these things can be shaped by religious views, which makes the candidate's religion relevant if they believe enough that their religion informs their worldview.