The Church then makes a pronouncement. If the apparition is of a supernatural origin it is told to the people that it is 'worthy of belief.' One is free to believe or not. We are not mandated to believe in any apparitions. However the overwhelming (in my mind) evidence for many of these apparitions convinces me.
Think of this also. What is a better way for the devil to have people think all this is kooky? Have people believe they are seeing something when in fact they are not. The devil as you are aware, is a "false coiner", a counterfiter. After and during every appartion that I've read about, satan tries to lure the people away with falsehood and his own trickery. Kind of like Slick Willie, smoke and mirrors to dispense with the truth.
But it could never work the other way. Satan could not possibly be trying to lure people INTO these apparitions, right?