[S]olicitude for all the churches has been committed to the holy Roman church, in Peter, the prince of the apostles. [Hincmar of Reims, On the Rights of Metropolitans, 4; quoted in Pelikan, The Growth of Medieval Theology, 48]Among other things, Hincmar said
The church of Rome was 'the mother and the teacher [mater et magistra],' whose authority was to be consulted on all questions of faith and morals, and her instructions were to be obeyed. [Pelikan, ibid.]Recall that he said this not as a fashioner of theological novelties and fancies, but like his fellows a man committed to preservation of the apostolic faith. Among other things, Hincmar also understood that only the Church could establish the canon of Scripture (ibid.).
And of course there is really no other possible means by which the canon could be decided. The Bible did not drop out of the sky, and it does not contain a list of canonical books itself. I don't remember the book, but A.B du Toit argued, in a work on the New Testament, that the canon is self-authenticating. I remember at the time, as a fervent presuppositionalist of the Van Til sort, that of course this had to be so.
How silly I was! How, pray tell, would a canon authenticate itself? Well, it can't. Many books (as for the Mormons, for example) claim that they are scripture, but of course that doesn't make it so. No, there is only one institution that can make such judgments, and that is the Church. Hincmar was right. God has not provided us a written list, and he does not gives us the canon by any other means than through the Church. The Holy Spirit has not spoken in audible voice to define for us the boundaries of Scripture, and any interior witness would be unreliably subjective.
And this is one reason why we must have the visible institution of the Catholic Church, as opposed to the invisible church posited by Protestants who reject that visible institution. Apart from a faithful Church - one that has stood firm throughout the millennia in proclaiming the truth of the Gospel - we would be faced with the same problem of subjectivism in determining the canon: where shall we find this invisible church of which they speak, how shall we know its members, and how shall it determine for us what the canon is? The answer is that we could not find such a church, we could not determine who is in it, we could not determine who is in a position to speak for it, and (for these reasons) we could have no confidence in its decrees concerning the canon.
We need the Church, and we need the Scripture, and we need Sacred Tradition. It's not an either/or proposition.