Previous to the reign of Constantine, or to the union of Church and State, heresy and spiritual offenses were punished by excommunications only; but shortly after his death capital punishments were added. Theodosius is generally allowed to have been the first of the Roman Emperors who pronounced heresy to be a capital crime. But the inquisitors at that time did not belong to the clerical order, they were laymen appointed by Roman prefects. Priscillian, the Spanish heretic, was put to death about 385. Justinian in 529 enacted penal laws against heretics, and as centuries rolled onward, the proceedings against them were marked by increasing severity. It was not, however, as we have just seen, until the thirteenth century that the court of Inquisition was established by canon-law. Then it became a criminal tribunal, charged with the detection, prosecution, and punishment of heresy, apostasy, and other crimes against the established faith. Whether Dominic or Innocent is to have the credit of the invention, it evidently had its origin in the Albigensian war. The papal legate discovered that the open slaughter of heretics would never accomplish their utter extermination. This difficulty led to the creation of a new fraternity, called the order of the Holy Faith; the members of which were bound by solemn oaths to employ their utmost powers for the repression of free inquiry in matters of religion and for maintaining the unity of the faith, for the destruction of all heretics and for the rooting out of all heresy from the homes, the hearts, and the souls of men. But it was reserved for Gregory IX., in the Council of Toulouse, to fix the establishment of the Inquisition in the form of a tribunal, and at the same time to give it positive laws.
This terrible tribunal was gradually introduced into the Italian states, into France, Spain, and other countries; but into the British islands it never was allowed to force its way. In France and Italy it required strenuous and persevering efforts to organize and establish it; Germany successfully resisted a permanent Inquisition; in Spain, however, though it met with some opposition at first, it speedily gained a footing, and in time attained a magnitude which, from a variety of causes, it never reached in any other country.
Gradually the authority of the inquisitors was extended, and they were called upon to pronounce judgment, not only upon the words and actions, but even upon the thoughts and intentions of the accused. During the fourteenth century, its progress was steady, whilst its rigor and energy were continually on the increase. But it was not till the close of the fifteenth century; when Isabella, wife of Ferdinand of Arragon, had ascended the throne of Castile, and when the different kingdoms of Spain—Castile, Navarre, Arragon, and Portugal—were united under these sovereigns, that the Inquisition became general in the country, and assumed that form which it retained until the period of its dissolution in 1808.