Keith Mathison argues that the doctrine of Sola Scriptura, as understood by the Reformed tradition, bears little re-semblance to the "Solo Scriptura" of evangelicalism. Far from advocating "no creed but the Bible," believers in Reformation churches insist that the Bible is the church's book, and must be interpreted by the church within the hermeneutical bounds of the regula fidei, the apostolic rule of faith that existed before the canon of Scripture was completed.
What place, then, is there for creeds and confessions?
Aside from their inevitability (all Christians believe something, whether written down or simply made up and stored in their "heart of hearts"), creeds are necessary to give light and expression to the teachings of Scripture. The doctrine of the Trinity, for example, gains creedal status in the Apostles' and Nicene creeds. "Why not just believe the doctrine because it is biblical?" we are often asked, "Why the need to make it creedal?" The answer is that without the hard work of those who wrote these creeds it is unlikely that every individual believer would arrive at the difficult yet biblical doctrine that God exists in three divine Persons, Father, Son, and Holy Spirit.
"But," the Catholic will object, "you just gave away the farm! By citing Nicaea you have effectively betrayed Sola Scriptura." To this the Reformed believer will respond by pointing out that the authority that our creeds and confessions carry is real, albeit secondary, penultimate, and derivative. In other words, the ultimate reason we hold to this or that creed is because it comports with the church's reading of Scripture as it is interpreted within the apostolic regula fidei, and not simply because it is a creed written by a lawfully-called church council.
This gives rise to a question for all my new Catholic friends: Why do you believe the conclusions of the Council of Trent? "Well," you might say, "because the conclusions reached at Trent were a conciliar statement of the one true Church." But even if I grant this for the sake of argument, I still want to know why you as an individual Catholic believe its decrees. Did you just get the memo and believe whatever it said before you read it, or did you seek to become personally convinced by history and Scripture that Trent was correct? Is this way of thinking "too Protestant"?
If a Catholic holds Trent's conclusions to be important because they represent the teaching of their church, but if a further layer of importance is added when he becomes convinced of those conclusions himself, then what I want to know is: Are you really that different from us, methodologically speaking?