This collection, indeed, comprises at least as many canons of councils as decretals, and the decretals contained in it are not all forgeries.
It is an amplification and interpolation, by means of spurious decretals, of the canonical collection in use in the Church of Spain in the 8th century, all the documents in which are perfectly authentic.
In the third part the author continues the series of decretals which he had interrupted at the council of Nicaea.
But as the collection of authentic decretals does not begin till Siricius (385), the pseudo-Isidore first forges thirty letters, which he attributes to the popes from Silvester to Damasus; after this he includes the authentic decretals, with the intermixture of thirty-five apocryphal ones, generally given under the name of those popes who were not represented in the authentic collection, but sometimes also under the names of the others, for example, Damasus, St Leo, Vigilius and St Gregory; with one or two exceptions he does not interpolate genuine decretals.
Back further than 847, the date of the False Capitularies, with which the author of the False Decretals was acquainted.'