It was one thing to bring a suspect before a judge; it was quite another to establish his guilt or innocence. In the Middle Ages with the decentralization of government, justice was administered by hundreds of independent feudal barons, counts, lords; it was local, crude, weak, selfish. In such circumstances people could not see why a judge, a man like themselves, should have superior power to declare them guilty of a crime. This difficulty was overcome by an appeal to God for adjudication. It appears that Germanic tribes had adopted a number of trials, particularly of personal combat, to determine justice in a dispute. With their conversion to Christianity it came but naturally to their minds to use these and other trials to appeal directly to God to make known His decision. The people of the Middle Ages believed firmly in God and were quite willing to accept His judgement in criminal cases. Thus the facts of the case, undetermined, were superceded by “the judgment of God,” the ordeals. […]
The Church never looked with favor on the so-called “common purgation,” i.e., ordeals, and has specifically forbidden judicial duels long since. The early popes terms them “temptation of God” and “superstitious inventions”; but they were supported by individual bishops and prelates, and Charlemagne was enthusiastic about their use. However the Church did make extensive use of “canonical purgation,” the taking of an oath as to one’s innocence, and the summoning of co-swearers regarding the integrity and reliability of the accused. It was employed when the charges against a person remained unproven, and was considered a supplementary proof. Thus when a bishop was charged with a serious office and conclusive proof was lacking, he could be ordered to purge himself by taking an oath that he was telling the truth, and provide a number of bishops or abbots to swear that he was worthy of belief.
Albert Shannon, The Medieval Inquisition (1991)