Each detail of this procedure calls for a few words of explanation.
The Inquisitor first summoned every heretic of the city to appear before him within a certain fixed time, which as a rule did not exceed thirty days. This period was called "the time of grace" (tempus gratiæ). The heretics who abjured during this period were treated with leniency. If secret heretics, they were dismissed with only a slight secret penance; if public heretics, they were exempted from the penalties of death and life imprisonment, and sentenced either to make a short pilgrimage, or to undergo one of the ordinary canonical penances.
If the heretics failed to come forward of their own accord, they were to be denounced by the Catholic people. At first the number of witnesses required to make an accusation valid was not determined; later on two were declared necessary. In the beginning, the Inquisition could only accept the testimony of men and women of good repute; and the Church for a long time maintained that no one should be admitted as an accuser who was a heretic, was excommunicated, a homicide, a thief, a sorcerer, a diviner, or the bearer of false witness. But her hatred of heresy led her later on to set aside this law, when the faith was in question. As early as the twelfth century, Gratian had declared that the testimony of infamous and heretical witnesses might be accepted in trials for heresy.[1]
[1] Pars ii, _Causa_ ii, quaest. vii, cap. xxii; _Causa_ vi, quaest. i, cap. xix.
The edicts of Frederic II declared that heretics could not testify in the courts, but this disability was removed when they were called upon to testify against other suspects.[1] In the beginning, the Inquisitors were loath to accept such testimony. But in 1261 Alexander IV assured them that it was lawful to do so.[2] Henceforth the testimony of a heretic was considered valid, although it was always left to the discretion of the Inquisition to reject it at will. This principle was finally incorporated into the canon law, and was enforced by constant practice. All legal exceptions were henceforth declared inoperative except that of moral enmity.[3]
[1] _Historia diplomatica Frederici II_, vol. iv, pp. 299, 300.