It happens every year. In teaching my humanities class, I ask what a philosopher had in mind in writing about the immortality of the soul or salvation, and suddenly my normally loquacious undergraduates start staring down intently at their notes. If I ask them a factual theological question about the Protestant Reformation, they are ready with an answer: predestination, faith not works, etc.

But if I go on to ask them how one knows in one’s heart that one is saved, they turn back to their notes. They look anywhere but at me,...