Ecclesiastical law was profoundly influenced by Roman law in both its procedural and substantive aspects. Not infrequently, indeed, extensive passages from the Corpus iuris civilis were incorporated into the canons verbatim. Accusatio, denunciatio, and inquisitio were the three methods of procedure employed in criminal trials. It was the last of these methods that the Church adopted with special modifications for extra-ordinary cases, the trial of heretics, and it was from the use of this judicial procedure that the institution of the Inquisition obtained its name. The first two methods were hopelessly inadequate for several reasons, but principally because of the lack of a public prosecutor. Nonetheless the were long preferred by lay courts even after the adoption of the procedure d’enquete by Louis IX. So far as secular criminal trials were concerned, the long tradition of the Germanic elements of lex talionis and ordeals, in spite of their condemnation by the Fourth Lateran Council of 1215, died but slowly. In the opinion of many thoughtful legal historians, the Church took a step forward when it substituted the inquisitio for the more primitive means, of the barbarians, of ascertaining guilt or innocence.
For the Inquisition was a court of exception, of extraordinary and summary procedure, adopted in order to repel a virulent, pressing danger. Even circumscribed as it was with various legal restrictions in regard to the defense of the accused, it established definite rules for gathering and sifting evidence and provided a specially delegated prosecutor and judge to pass on the testimony so presented. In the early days of the functioning of the Inquisition as organized by Pope Gregory IX it would appear that all the rights of Canon Law were allowed to the defense, including the revelation of the names of those bringing the charges. Upon arraignment the accused was informed of the suspicions, denunciations, and charges that had been entered against him. However, with the growth and extension of the operations of the inquisitors and the experience gained thereby, it became increasingly evident that some protection must be afforded the witnesses, or else no one could be prevailed upon to testify…That witnesses were in imminent danger is an historical fact, and both ecclesiastical and lay courts so recognized it.
Albert Shannon, “The Secrecy of Witnesses in Inquisitorial Tribunals and in Contemporary Secular Criminal Trials”, Essays in Medieval Life and Thought: Presented in Honor of Austin Patterson Evans (1965)