Mark's reply immediately follows Athanasius's request. At first he has to clear his throat. Mark says he's sorrowed to hear Athanasius and company are having a hard time, but nonetheless consoled because the Egyptian bishops are holding fast to the faith. "You are not a reed shaken with the wind," he says, nodding at Matt. 11:7. Then Pseudo-Mark borrows from a letter of Leo the Great to say that he recognizes the strength of his correspondents' constancy, and that he is happy they're exercising their vigilance for Christ's flock; this is important so that the wolves in sheep's clothing can't get at the simplices. Athanasius and his friends should keep up the good work, for those who persevere to the end are blessed for their patience. According to the Gospel, persecution accompanies whoever desires to live piously in Christ.
With that out of the way, Pseudo-Mark hops along to the interesting bit. He says that he's investigated Athanasius's claims about the Council of Nicaea by consulting witnesses who attended the proceedings. These, of course, confirmed Athanasius's account. Mark hastens to reassure his friend that he didn't go to all this trouble because he was worried Mark was putting him on -- he just needs to keep the church united, out of trouble, free from the insult of heretics, immaculate, firm and immobile for all time. You can tell Pseudo-Isidore is writing in his own words here: Few other writers, forgers or polemicists or otherwise, get worked up enough to lay such stress on the unity and orthodoxy of the Roman church.
Only after these inquiries did Mark bother to open up his filing cabinet. Tucked away in his scrinium, he found the seventy chapters Athanasius was talking about, as directed to his predecessor Sylvester eleven years previously in 325. Sort of makes calling in those witnesses pointless, doesn't it? After all, if Mark only had to root around in the archives to verify that the Egyptians weren't full of it, why bother?
Anyway, Mark is a generous guy but he's not going to send Rome's only copy of the Nicene canons to Egypt. The document possesses great authority as a witness to the council, and it has a funky subscription list besides (from which remarks we are meant to understand that the pope hasn't just got any old copy, but is in fact looking at the original acta as promulgated at Nicaea). Thus he's had a copy made in the presence of Athanasius's messengers, who can bring it back to the land of canon-burning heretics without endangering the original. This copy, Mark hastens to confirm, has the same number of canons, the same words and the same subscription list as Mark's. Our letter does nothing if not protest too much.
The pope pinches off his epistle with a long passage on heresy (and the purity of the Apostolic Church therefrom) that we also find concluding the only letter of Lucius (blogged previously) and the third letter of Felix, both forgeries from Part I.
Recipients: Athanasius and the rest of the Egyptian episcopate
Date: 24 Oct. 336 ("Nepotiano et Fecundo viris clarissimis consulibus": the only set of consuls on offer in the Liber Pontificalis)
Sources: the Bible (Matthew), letters of Leo the Great and Celestine I (both from the Hispana, presumably), the Concilium Africanum of the Dionysio-Hadriana, the Liber Pontificalis (only for the consuls)