The papal Inquisition, properly so-called, originated with Pope Gregory IX (1227-1241). The Inquisition was an extraordinary court of exception established by the papacy to investigate and adjudicate persons accused of heresy. It was a rational inquiry by special officials appointed by and responsible to the Holy See. The purpose was to bring order and legality to the procedure for dealing with heresy, since, on the one hand, the attitude in southern France was one of benign indifference or even approval of heresy, while in the north, on the other hand, particularly in Germany, there was the tendency to burn alleged heretics forthwith. While the inquisitors worked in cooperating with the local bishops, their authority derived from Rome, which now chose to exercise primary jurisdiction over heresy and heretics. Although many profess to see the origin of the Inquisition in previous Councils and synods, these regulations pertained to the ordinary duties of the local bishops, who always had this responsibility along with all the other demands of their office. Now for the first time, Gregory IX created a new procedure whereby an office was established whose primary and only function was the searching out of heresy and the resolution of the findings. It was both a new procedure, a rational inquiry by a judge, and a new institution, a papal agency designed to utilize this new procedure for uncovering and trying alleged heretics.
The pope was quite well aware of all the efforts that had gone before – and that they had not succeeded. Now, with conditions considerably modified, he was able to enact legislation himself with the assurance of having the active support of the secular powers and the aid of the newly founded mendicant orders, the Dominicans and the Franciscans. The consequent close association and correspondence between the pope and the inquisitors in the field made for the gradual crystallization of a definite procedure and clearly defined juridical processes, The stress and strain of meeting and dealing with individual problems that arose out of the trials of heretics necessitated the sending of inquiries to Rome for solution. It was from the replies of the popes and of regional meetings of bishops to these questions of the inquisitors that the legal code and form of the Inquisition developed.
Albert Shannon, The Medieval Inquisition (1991)