How do you tell a visionary from a nutcase, or even someone possessed by an evil spirit? The Vatican is instructing bishops on how to test the spirits in a new apparitions handbook, helping them deal with a phenomenon that has increased dramatically in recent years.

The first principle is that anyone claiming a vision should be told to keep quiet about it until it's approved. If they disobey the embargo, this will be taken as evidence of inauthenticity.

The ...