Christine Caldwell Ames describes the way heresy trials changed over time.
What was so terrible about the Spanish Inquisition was that, again, it was good at what it did. And one point to make clear is that the Spanish Inquisition differed from medieval inquisitions in that it was a royal institution, it was a state institution. And again, this is something that historians try to be very careful about. Medieval inquisitions were ecclesiastical trials that, again, very often had the support, needed the support, of local secular authority. But the Spanish Inquisition was a state institution and what was bad about it – that is, what was effective about it – was that it worked very well.
And it has become most notorious particularly because it pursued people known as Conversos, that is, converted Jews. And certainly there has been a lot of scholarship, both popular and otherwise, that has examined the reality of this Converso population. And one of the great suspicions about the Spanish Inquisition which we don’t seem to see about medieval inquisitions is that inquisitors generated accusations, that is, they wanted to find people that they could suspect, people who would be guilty, for the very purpose of sustaining the institution. And so I think for a lot of people, the Spanish Inquisition is alarming because it seems to look forward to future institutions of supervision and punishment for their own sake.