"Why is this a peculiar RC mental condition?"

Not really. Luther, awakened by the Devil at night, had a number of battles with the Devil. He even threw an inkwell at his visionary or real Satan. And Joseph Smith, founder of the Mormons, had a series of visions of Jesus and God the Father, and had the Angel Moroni direct him to some "golden tablets." And what about Muhammad? He had the words of God revealed to him by the Angel Gabriel. Whether those private revelations are legit, produce legit miracles are up to the believer. But visions, private revelations are not just a Catholic "thing" in religious history or religious studies. They seem to have a double existence in many faiths. But no one is forced to believe in any vision or "hidden certainty" seen on some tree top or in a dime and penny shop window.