I agree that religion should be separated from government, but that is exactly why I think candidates should be asked about it. People have a right to know whether a presidential candidate believes in the Holy Bible. If any presidential candidate takes the Bible as literal truth, it is a cause for serious concern. The Bible is filled with horribly inaccurate moral and scientific statements, and anyone who believes them is not fit to lead a country.
Any moral decision that the president would make would be affected by his religious views. A person's opinions concerning abortion, assisted suicide, death penalty, or drug use could all be impacted by the irrational propositions he finds in an ancient book.
Faith in general is not something that should be desired in a person that is expected to be rational and open-minded.