I googled ‘false decretals’ and got a couple of replies which may be of background use for you.
Why waste your time on Protestant websites with a group of people who have nothing better to do then dredge up spurious attacks on Catholicism. If they were seekers after the Truth I could understand it but these people are obviously anti Catholics who have no interest in converting. Wouldn’t you be better off developing your interior life by reading good solid Catholic material? If you come up with an answer to this objection they will just come up with another objection and so on. You will spend your time finding answers they don’t really want to hear.
When I first got on to the computer I got into this ‘justify your Faith’ bandwagon until I woke up to myself and now I spend the time my time more profitably developing my interior life and praying for those people who in their heart of hearts know that there is something missing from their belief system - the Truth of Catholicism.
In This Rock magazine (online) there is an article by Steven O’Reilly in their October 1998 issue entitled ‘The False Decretals’ and the Catholic Encyclopedia (online) has quite a lengthy article on the False Decretals. Below is part of the article from CE
(The Decretals of the Pseudo-Isidore)
False Decretals is a name given to certain apocryphal papal letters contained in a collection of canon laws composed about the middle of the ninth century by an author who uses the pseudonym of Isidore Mercator, in the opening preface to the collection.The name “False Decretals” is sometimes extended to cover not only the papal letters forged by Isidore, and contained in his collection, but the whole collection, although it contains other documents, authentic or apocryphal, written before Isidore’s time.
The Collection of Isidore falls under three headings:
(1) A list of sixty apocryphal letters or decrees attributed to the popes from St. Clement (88-97) to Melchiades (311-314) inclusive. Of these sixty letters fifty-eight are forgeries…
(2) A treatise on the Primitive Church and on the Council of Nicæa, written by Isidore, and followed by the authentic canons of fifty-four councils…(3) The letters mainly of thirty-three popes, from Silvester (314-335) to Gregory II (715-731). Of these about thirty letters are forgeries, while all the others are authentic. .
THEIR APOCRYPHAL CHARACTER
Nowadays every one agrees that these so-called papal letters are forgeries. These documents, to the number of about one hundred, appeared suddenly in the ninth century and are nowhere mentioned before that time. The most ancient Manuscripts of them that we have are from the ninth century, and their method of composition, of which we shall treat later, shows that they were made up of passages and quotations of which we know the sources; and we are thus in a position to prove that the Pseudo-Isidore makes use of documents written long after the times of the popes to whom he attributes them. Thus it happens that popes of the first three centuries are made to quote documents that did not appear until the fourth or fifth century; and later popes up to Gregory I (590-604) are found employing documents dating from the sixth, seventh, and eighth centuries, and the early part of the ninth. Then again there are endless anachronisms. The Middle Ages were deceived by this huge forgery, but during the Renaissance men of learning and the canonists generally began to recognize the fraud.