I just want to talk a little more about the passage that concludes Mark's letter, which I said recurs in two other forged decretals. I'll call it the Haec apostolorum from its incipit. Its contents come primarily from the acta of the Third Council of Constantinople, with some lines from a letter of Leo the Great (ep. 7) and Benedictus Levita (III.55) thrown in. It asserts that the apostolic tradition is to be cultivated and adhered to by everyone; the popes have never erred and will never fall into heresy. That which previous pontiffs enacted is thus to be remembered. The writer fears what will happen to him if he neglects to preach the "truth of our savior Jesus," or if he oppresses that truth with silence. Thus, the pope's correspondents are advised to be on the lookout against heretics and enemies of God's church, and to attack any heresy with as much severity as possible.
Every time I run into one of these recurrent passages I find myself wondering whether they were originally developed for one forged letter and later transferred to others, or whether they're separately-composed snippets designed for repeated use. I wonder, in other words, to what degree the Pseudo-Isidorian workshop assembled letters from prefab parts, and to what degree they attended to each as a separate, coherent composition designed to make a discrete argument. (In which connection see also my previous obsessive musings on cohesiveness.) In the case of Haec apostolorum, I can't make any solid argument about an original environment, but it seems to fit best with the Mark letter.
At first glance it looks fine in Lucius. That letter starts out with some bellowing about the persecution of faithful bishops at the hands of heretics, and though it goes on to talk mainly about (you guessed it) episcopal accusations, Haec apostolorum brings the focus back to heresy at the end. This basic correspondence between the opening and closing passages of a decretal -- frequently with sharply different intervening material -- is a common feature of the forgeries that we've observed before. Yet there's no more specific resonance and no shared source material. That is, none of the sources employed in Haec apostolorum recur in earlier passages of the Lucius letter. That's not all that unusual, but without any more explicit contact it becomes hard to argue that our passage was crafted specifically for Lucius.
You could argue that the text is a little less at home in the third letter of Felix (not yet blogged but edited here). Beyond Haec apostolorum, this decretal consists almost entirely of one long passage clipped from Idacius's anti-Arian treatise Adversus Varimadum. To the extent this letter is about heresy, our passage is at least vaguely on point, though again there's no correspondence at the level of sources. The last Felix letter also falls pretty clearly into the category of filler; our forgers are obviously putting very little work into most of its text. I guess they could've woken up at the end and decided to more tightly construct its closing lines, but that would be unusual in the case of emptier letters, like this one, that lack an historical hook or any overt arguments on Pseudo-Isidore's pet htemes.
The thing that really jumps out at me when I consider Haec apostolorum in the context of the just-blogged Mark letter is the similarity between aspects of this text and the Lucius forgery. Both letters have vastly different contents, of course, but both pretend to have been prompted by similar episcopal complaints. Though Pseudo-Isidore doesn't bother to invent whatever letter Lucius is supposed to be responding to, Lucius's address and initial lines discuss bishops "in Gaul and Spain" whose "afflictions" have saddened the pope. The bishops have been "persecuted and defamed" by those who do not adhere to the correct faith; their churches have been robbbed of their possessions and despoiled of the offerings of the faithful, to the point that the persecuted can neither defend themselves nor offer necessary services to the faithful. All of which is redolent of Athanasius's complaint that heretics have destroyed all his stuff, to the point that he has neither books, nor ecclesiastical vestments, nor ornaments nor other utensils (presumably meaning altar vessels).
Haec apostolorum concludes Pope Mark's reply. While in Lucius and Felix there was general thematic corresponsence with earlier passages, here it explicitly interacts with the argument (though once again it shares no sources with the rest of Mark's letter). At the beginning of the decretal, Pseudo-Mark justifies his investigation of Athanasius's claims by declaring that the Roman church "has remained and will remain...forever unstained," immobile and free of any heresy. This is just an extremely emphatic rewording of a line from Haec apostolorum, which declares that "This holy and apostolic church, the mother of all churches...which will be shown never to have erred, has not succumbed to heretical innovations...."
Anyway, that's enough for now. Someday I hope to be able to post a broader survey of these recurring passages -- there's not all that many of and they're easy enough to find with Hinschius's apparatus fontium).