This distressing finding comes from a long study done by Bette L. Bottoms at the University of Illinois at Chicago. She directed two separate national surveys, combining data from over two thousand mental health professionals and over four hundred police agencies. Most cases involved corroborating evidence or an admission of guilt.
She found that although the majority of the abuse cases involved priests, by no means were they the only perpetrators. Other findings that go against the popular suspicion include the fact that girls are often the victims of these crimes as well. And sexual abuse was only about equally as common as other forms of abuse, such as severe physical punishment.
Finally, Bottoms notes that her surveys found zero instances of Satanic cults abusing children. There was a time back in the nineties when the idea that Satanic cults existed and were abusing children was widespread, but the data don't back this up--though the actual facts are even more sobering.