Canon law, helped greatly by the revival of the imperial code, introduced in the twelfth century definite laws for the suppression, of heresy. This régime lasted from 1150 till 1215, from Gratian to Innocent III. Heresy, the greatest sin against God, was classed with treason, and visited with the same penalty. The penalty was banishment with all its consequences; i.e., the destruction of the houses of heretics, and the confiscation of their property. Still, because of the horror which the Church had always professed for the effusion of blood, she did not as yet inflict the death penalty which the State decreed for treason. Innocent III did not wish to go beyond the limits set by St. Augustine, St. John Chrysostom, and St. Bernard. But later Popes and princes went further. They began by decreeing death as a secondary penalty, in case heretics rebelled against the law of banishment. But when the Emperor Frederic had revived the legislation of his Christian predecessors of the fourth, fifth, and sixth centuries,[1] and had made the popular custom of burning heretics a law of the empire, the Papacy could not resist the current of his example. The Popes at once ordered the new legislation vigorously enforced everywhere, especially in Lombardy. This was simply the logical carrying out of the comparison made by Innocent III between heresy and treason, and was due chiefly to two Popes: Gregory IX who established the Inquisition under the Dominicans and the Franciscans, and Innocent IV who authorized the Inquisitors to use torture. [1] Cf. the law of Arcadius of 395 (_Cod. Theodos_., xvi, v. 28). The theologians and casuists soon began to defend the procedure of the Inquisition. They seemed absolutely unaffected, in theory at least, by the most cruel torments. With them the preservation of the orthodox faith was paramount, and superior to all sentiment. In the name of Christian charity, St. Thomas, the great light of the thirteenth century, taught that relapsed heretics, even when repentant, ought to be put to death without mercy. How are we to explain this development of the doctrine of the Church on the suppression of heresy, and granting that a plausible explanation may be given, how are we to justify it? . . . . . . . .