I had to trust the Church as reliable witness to the canon of Scripture

James Akin in http://www.ewtn.com/library/ANSWERS/AKINSTOR.htm

The Protestant doctrine of sola scriptura ... began to trouble me as I wondered how it is that we can know for certain which books belong in the Bible. Certain books of the New Testament, such as the synoptic gospels, we can show to be reliable historical accounts of Jesus' life, but there were a number of New Testament books (e.g., Hebrews, James, 2 Peter, 2 and 3 John, Jude, and Revelation) whose authorship and canonical status were debated in the early Church. Eventually the Church decided in their favor and included them in the canon of inspired books, but I saw that I, a person two thousand years removed from their writing, had no possibility of proving these works were genuinely apostolic. I simply had to take the Church's word on it.

This meant that for one very foundational doctrine-the doctrine of what Scripture is-I had to trust the Church since there was no way to show from within Scripture itself exactly what the books of the Bible should be. But I realized that by looking to the Church as an authentic and reliable witness to the canon, I was violating the principle of sola scriptura. The "Bible only" theory turned out to be self-refuting, since it cannot tell us which books belong in the Bible and which don't!

What was more, my studies in Church history showed that the canon of the Bible was not finally settled until about three hundred years after the last apostle died. If I was going to claim that the Church had done its job and picked exactly the right books for the Bible, this meant that the Church had made an infallible decision three hundred years after the apostolic age, a realization which made it believable that the Church could make even later infallible decisions, and that the Church could make such decisions even today.