Judicial agency created under papal authority for the discovery, prosecution, and punishment of heresy and certain other acts defined as criminal under canon law, such as bigamy and sodomy. Christianity had always exercised the power to expel members who rejected its doctrines or moral requirements. After it became the official religion of the Roman Empire during the fourth century, imperial legislation made the local bishops judges over their community. In the first half of the Middle Ages, the bishop or his judicial deputy dealt with cases of heresy but never claimed the right to take a defendant's life. In the early 13th century, after the suppression of the Albigensian heresy in southern France by a crusade authorized by Pope Innocent III, the Roman curia concluded that the local bishops had been too lax. Since the Albigensian heretics of southern France remained stubborn and elusive, in 1233 Pope Gregory IX began appointing special inquisitors in some regions, charged with the duty of suppressing heresy.
Although the procedures followed by these inquisitors seem harsh when evauated by modern standards, they were modelled on the practices of Roman law. An inquisitor began by publicizing a grace period of 30 days during which heretics could identify themselves, abjure their errors, and be reconciled to the church. After this interval, all good Christians were supposed to denounce known heretics. Because of fear that powerful individuals would intimidate potential accusers, the names of accusers and their testimony were kept secret. Accused persons were, however, invited to present a list of personal enemies who might have made false accusations. In cases where there was substantial suspicion of heresy, the judges had the authority to interrogate the accused under torture, another procedure borrowed from Roman law. Inquisitors were fully aware that using torture could be a means of getting the testimony one wanted rather than getting the truth. Torture was regarded as a last resort, but it was used.
Being charged before an inquisitorial tribunal was a serious matter. Yet a substantial proportion of defendants was acquitted. Of those found guilty, most were assigned modest penalties and then released under supervision. For a person who had been condemned previously and had relapsed into heresy, however, a sentence of death was virtually certain. Although theoretically, being a church court, the inquisitorial court did not execute condemned heretics, in such cases guilty persons were "relaxed to the secular arm"—that is, to the secular government—which was then expected to carry out the execution.
Although historians have often used the term "inquisition" loosely, the medieval inquisition was not a formal institution but merely a number of individuals (usually Dominican or Franciscan friars) commissioned to investigate and punish heresy in a region. They were not active in all parts of Europe, and where they were active their efficacy was often questionable, though they did succeed in exterminating the Albigensian heresy. There was no institutional network, no systematic communication between inquisitors, and no system of supervision to ensure that their work was effective or free from abuses. In addition, since diocesan bishops still retained their own judicial authority, there were often conflicts of jurisdiction in the investigation and prosecution of heretics.
Historical Dictionary of Renaissance. Charles G. Nauert. 2004.