The "Catholic Church" didn't exist until some time after Constantine converted in 312 AD, However....
Around 250 A.D., Origen likely produced a complete list of all 27 New Testament books–more than a hundred years before Athanasius. In his typical allegorical fashion, Origen used the story of Joshua to describe the New Testament canon:
But when our Lord Jesus Christ comes, whose arrival that prior son of Nun designated, he sends priests, his apostles, bearing “trumpets hammered thin,” the magnificent and heavenly instruction of proclamation. Matthew first sounded the priestly trumpet in his Gospel; Mark also; Luke and John each played their own priestly trumpets. Even Peter cries out with trumpets in two of his epistles; also James and Jude. In addition, John also sounds the trumpet through his epistles [and Revelation], and Luke, as he describes the Acts of the Apostles. And now that last one comes, the one who said, “I think God displays us apostles last,” and in fourteen of his epistles, thundering with trumpets, he casts down the walls of Jericho and all the devices of idolatry and dogmas of philosophers, all the way to the foundations (Hom. Jos. 7.1).
As one can see from the list above, all 27 books of the New Testament are accounted for (Origen clearly counts Hebrews as part of Paul’s letters). The only ambiguity is a text-critical issue with Revelation, but we have good evidence from other sources that Origen accepted Revelation as Scripture (Eusebius, Hist. eccl. 6.25.10).
My thoughts: The early church leaders (disciples of the apostles) wrote a lot of letters to each other, mostly to deal with heresies. In those letters, as a whole, a person can reconstruct about 2/3 of the traditional New Testament canon either quoted or paraphrased. Thus, it is easy to see which writings were used as authoritative (as well as what was, and what was not "sound doctrine")..
The appearance of "a list" by Athanasius in 380 AD or so, has little to do with which letters/books were accepted as authoritative by the first congregation elders centuries earlier.