The Pseudo-Isidorian Decretals and certain fictitious letters ascribed to early popes, from Clement to Gregory the Great were incorporated in a ninth-century collection of canons purporting to have been made by a certain, apparently fictitious, Isidore Mercator, not to be confused with the early medieval encyclopedist Isidore of Seville. The useful name "Pseudo-Isidorian Decretals" has been in common use since the awakening of textual criticism among humanists of the 16th century. Since the decretals and letters are included with spurious Hispanic canons and other forgeries, the critical editor Bernhard Eduard Simson in 1886 gave the fitting designation "Pseudo-Isidorian Forgeries" to the whole series.

A measure of the widespread usefulness that the collections presented can be judged by the fact that seventy-five manuscripts of the Pseudo-Isidorian material have survived, which differ widely one from another. Collections of canons were commonly made by adding new matter to old. The forger of the Pseudo-Isidore collection took as the basis of his work a quite genuine collection Hispana Gallica Augustodunensis and interpolated his forgeries among the genuine material.

The official Liber pontificalis was used as a historical guide and furnished some of the subject-matter. The most notorious of the forgeries in the Pseudo-Isidorian collection is the Donation of Constantine. The falsity of the Pseudo-Isidore's fabrications is now admitted; proved by incontestable internal evidence such as the anachronistic use of the language of the Vulgate and of the Breviarium Alaricianum (written in 506) in the decretals of earlier popes. The Pseudo-Isidorian letters were unknown before 852 or 857.

Immense labor and erudition went into creating this work, and a wide range of genuine sources were employed. The earliest use made of the Pseudo-Isidore material dates to the 850s, giving a terminus before which the collection must have been made. The general agreement is that the work had its origin in the Kingdom of the Franks.

The forger's main object was to emancipate bishops, not only from the secular power, but also from the influence of archbishops and synods, partly by exalting the papal power. The uses made of the forgeries form a historical study in themselves.

A section from a spurious letter purporting to be from Jerome to Pope Damasus is at the entry Pope Damasus I.

External link

• Christian Classics Ethereal Library: Pseudo-Isidorian Decretals and other forgeries