Numerous questions and problems arise from the fact of private revelation. The first and most obvious one is that a great many alleged private revelations are fake or false. Note the distinction between “fake” and “false.” A fake private revelation is a deliberate deception. An average Evangelical (and, for that matter, an average Catholic) is typically ready to assume that a claimed private revelation is fake, and there is good reason for that. The world abounds with charlatans claiming miraculous powers and looking for fame, money, sex, or power. They are found in all religious traditions. The rule of thumb regarding fake revelation is: There’s a sucker born every minute. Don’t be one of them. Trust God and keep an eye on your wallet.
But keep an even closer eye on the teaching of the Church. Not all fake private revelation is after your wallet. Sometimes it’s after your soul. The devil does indeed come to us as an angel of light. And he always seeks to turn us away from the teaching of Holy Church toward the worship of some creature (it matters little which creature). Therefore, a private revelation that sets itself up against the public revelation of the Church or the authority of her pastors is, by definition, not to be accepted, because God cannot contradict himself.
But not all false private revelation is fake. A “revelation” can be false although the person experiencing it may seriously believe it’s legitimate. In such a case, the person claiming the revelation isn’t a crook or a liar, he’s just mistaken. But sincerity doesn’t guarantee immunity from the harm a false revelation may cause. People who mistake the exit ramp for the on ramp of the freeway also believe they’re going the right way. That doesn’t mean they don’t experience painful and even fatal consequences as a result of their error. People can be sincerely wrong.
For instance, somebody may take seriously a false revelation claiming that blood transfusions are sinful. As long as you and your loved ones are in good health, such a blunder is only a quirky notion. But if you (or worse still, someone in your care, like a child) are involved in a car accident, your commitment to a false revelation could spell the difference between life and death.
That said, there's another complicating factor: the weirdness of God's mercy. Of which more next time.