Also called Pseudo or False Decretals. A spurious compilation of fifty-nine decretals by Mentz, who lived in the ninth century, and fraudulently ascribed them to I'sidore of Seville, who died in the sixth century. Prior to the ninth century the only authentic collection of decretals or letters of the popes in reply to questions proposed to them by bishops, ecclesiastical judges, and others, was that of Dionysius the Little [Exiguus], a Roman monk, who lived in the middle of the sixth century. He commences with Pope Siricius (fourth century). The Isidorian decretals contain fifty-nine letters ascribed to persons living between Clement and Siricius, and forty others not contained in the Dionysian collection. The object of these forged letters is either to exalt the Papacy or enforce some law assuming the existence of such exaltation. Amongst these spurious letters are the decretal of St. Anacletus, the decretal of St. Alexander, the letter of Julius to the Easterns, the synodical letter of St. Athanasius, the decretal of St. Fabian instituting the rite of the chrism, and so on.
“La réforme pseudo-Isidorienne, adoptée par S. Nicholas, en 865, par
le huitieme concile oecumenique en 870, confirme par le concile de
Trent en 1564, elle est depuis neuf siecles le droit commun dans
l'eglise catholique ... ce qu'il est impossible de justifier et inérne
d'excuser, c'est le moyen employe par le pseudo-Isidore pour
arriver a ses fins.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894