As the Inquisition proceeded through the 1400s, its focus shifted from Jews and heretics towards so-called witches. Although Pope Gregory IX had authorized the killing of witches back in the 1200s, the fad just didn't catch on. In 1484, Pope Innocent VIII issued a bull declaring that witches did indeed exist and thus it became a heresy to believe otherwise. This was quite a reversal because in 906 the Canon Episocopi, a church law, declared that belief in the existence and operation of witchcraft was heresy.
Read Article: Witchcraft and the Inquisition: Using the Inquisition to Suppress Dissent & Outsiders