You're right, I should clarify: It is a common, comforting assumption I see Christians make that nobody sincerely disbelieves in Christianity, and that every supposed unbeliever privately prays to the god of the Bible in times of need.

This does not preclude the possibility that some people have done this. The point I want to make is the psychological pressure driving the assumption that all unbelievers do this, and that no actual unbelievers exist, just rebellious secret Christians.