Writing at The Catholic Thing, he says of his once held belief in sola Scriptura:
First, in defense of the Protestant Old Testament canon, I argued, as noted above, that although some of the Church’s leading theologians and several regional councils accepted what is known today as the Catholic canon, others disagreed and embraced what is known today as the Protestant canon. It soon became clear to me that this did not help my case, since by employing this argumentative strategy, I conceded the central point of Catholicism: the Church is logically prior to the Scriptures. That is, if the Church, until the Council of Florence’s ecumenical declaration in 1441, can live with a certain degree of ambiguity about the content of the Old Testament canon, that means that sola scriptura was never a fundamental principle of authentic Christianity.After all, if Scripture alone applies to the Bible as a whole, then we cannot know to which particular collection of books this principle applies until the Bible’s content is settled. Thus, to concede an officially unsettled canon for Christianity’s first fifteen centuries seems to make the Catholic argument that sola scriptura was a sixteenth-century invention and, therefore, not an essential Christian doctrine.
These are good points to remember when discussing the notion of 'Scripture alone' with others. Be sure to read the rest of his thoughts.Second, because the list of canonical books is itself not found in Scripture – as one can find the Ten Commandments or the names of Christ’s apostles – any such list, whether Protestant or Catholic, would be an item of extra-biblical theological knowledge.