Either you misunderstood what I wrote, or I wrote poorly. I agree that there was no universal papal authority in 400AD. That was an innovation the Bishop of Rome pushed in contradiction to the scripture. I also agree that at that time, there was no concept of a Christian who wasn’t Catholic. I also agree that there wasn’t total unanimity on the canon of the NT, although I believe there was a pretty strong consensus from the beginning that the gospels and the letters of Paul were inspired by God. But even the (Roman) Catholic Church allowed debate on canon until Trent.
My point was that someone who accepts current Catholic teaching - which evolves, as has been pointed out - sees meaning in Augustine (and scripture!) that wasn’t meant by the writer.
And avoiding that error is always a challenge for a modern reader. I’m sure I fail at it often myself.
Of course, being Latin, Blessed Augustine was jurisdictionally subject to the Bishop of Rome (that is—the Patriarch of the West).
When Blessed Augustine says he is moved by the authority of the Catholic Church to believe thesciptures, he is referring to the entire Church (the Greek and Latin side).
However, being Latin, he was jurisdictionally subject to the Bishop of Rome (that is—the Patriarch of the West), so, he might just as well be referring to the Roman Catholic Church.
But He is definitely not referring to the "community of believers," as some Protestants imply.