Torture itself was of Roman origin. Roman law in the days of the Republic permitted only a slave or a provincial to be tortured. However, in the early days of the Empire the custom was begun of subjecting to this process of examination a Roman citizen accused of treason. From then on, references to torture are numerous in the Roman codes, which came to be of general application. It is therefore not surprising that the diffusion of torture coincides with the Reception of Roman Law by the legists of the Bologna school, though some authors claim that the Germans customarily used torture even before the revival of Roman law in Europe. In any case, torture was customarily employed in secular courts in the high Middle Ages and remained as an integral part of criminal proceedings in the common law of Europe down to the French Revolution.
As far as canon law is concerned, torture entered by a different route. Though it was common in Europe it was not envisaged for ecclesiastical trials. By one of the quirks of legal history, which regards the wording of the law as sacrosanct, the process whereby a citizen of Rome accused of treason could be tortured, and, if guilty even be put to death, was taken up by Pope Innocent III for quite a different reason. He stated in his constitution of 1199 Vergentis in senium that since in common law persons guilty of the crime of treason were punished by death and the confiscation of their property, so much the more should those who strayed from the Faith and offended God be inflicted with ecclesiastical censure and confiscation of their property, for it was far more serious to injure divine majesty than human. The pope used this analogy to indicate why confiscation of heretics’ property was in accordance with common law. He never considered torture or the death penalty for heresy – quite the contrary. And in the final codification of his decrees in regard to heretics in the Fourth Lateran Council, no mention is made either of torture or the death penalty – nor were they employed by the Church.
Albert Shannon, The Medieval Inquisition (1991)