This is interesting. Paul’s epistles are not catholic epistles?
My alleged point is that the word "catholic" when applied to the Epistles of Peter, James, etc. does not mean "CATHOLIC" as opposed to Calvinist (or what have you) but "General" as opposed to "to Timothy" or "to the Church at Ephesus, or whatever.
When I was an Episcopalian and we announced a reading from, say, I Cor, it would be, "Here beginneth the x verse of the y chapter of the First Epistle of St. Paul to the Corinthians." But if it was, say, I Pet, it would be, "Here beginneth the x verse of the y chapter of the First Epistle General of St. Peter."
So "catholic" (and, Iscool, I have no idea whether and how the writers to whom you refer use majuscule and minuscule letters, and I don't care much either) in this instance has the root sense of "according to the whole [Church],"as opposed to "to a particular person or congregation."