Protestants confess that Scripture alone has ultimate objective religious authority in the life of the Christian and the church. This does not mean, however, that divine revelation comes only through Scripture, a common mistake made by some evangelicals. Though the Scriptures reveal Christ immediately, or directly, more subjective forms of authority have also had a proper and important place. This is often denied by the more rationalistic sorts of Reformed Christians.
I would most definitely include, in a proper understanding of authority, holy tradition. By this I refer to the Holy Spirit speaking to the church down through the ages in the various ways that Christians have heard him through Scripture leading the church and helping Christians reach consensus about important subjects. We also listen for the Spirit’s voice in reason, properly understood as thinking God’s thoughts after him and not as rationalism. The same is true in experience, where real change leading to obedience is the obvious result of the Spirit’s work upon us and the church. By experience we gain insight into God’s nature and our human conduct. This happens regularly, for example, as Christians reflect upon nature and natural revelation. (Non-believers cannot understand this revelation since they willfully reject it; cf. Romans 1:18-23.) Simply put, we can and should gain insight that does not come to us directly from Scripture. To deny this will have huge negative consequences. These consequences are most often seen in various rigid expressions of modern Protestant fundamentalism.
There are two ditches to be avoided. One ditch presents to us an opportunity to fall into a Scripture alone position that, in effect, acts as if God speaks to us only in Scripture. In this ditch the Bible becomes a collection of proof texts for all kinds of bizarre and suspicious conclusions but the interpreter is assured that he is correct since he always has the Bible on his side. No one need listen to any previous church teaching on the subject since we have the text to guide us perfectly. This is not the real Protestant view of sola Scriptura at all, though you could fool many proponents of the view. The other ditch we can fall into presents the very grave danger of basing revelation entirely, or finally, upon our thoughts, or experiences, without testing every word by the Scripture’s clear witness to Christ. (The Scripture is, in the proper Protestant view, the Supreme Court!) In this ditch the error is to reshape the Scripture to fit what we know or understand without a proper and careful appeal to the written Word. Scripture is the final word about Christ. But remember: It’s purpose is to show us how we go to heaven, not how we understand the heavens. (It’s purpose is to show us how to live a godly life but it is not the last word, for example, on medical science and how it might help us improve life. This is how certain kinds of Christian counseling ["so-called"] go awry.)
We must keep Scripture in its primary place without surrendering the gains of tradition, human insight and common experience. This process is never tidy and thus is precisely one of the major reasons why our Catholic friends reject our Protestant principle on authority and argue for a holy magisterium. (I respect their concerns profoundly but remain convinced of the Protestant principle, rightly defined and rightly used!) We would help the situation considerably if we stopped using a fundamentalist approach that does not give Christian tradition its proper place. One sure way to do that is to stop treating Catholic and Orthodox Christians and churches as something other than historically valid churches. We should not consign these older and historic churches to the "cult" bin the way fundamentalism has done for almost a century. (You can still raise a lot of money for your cause, at least in many Protestant corners, if you are anti-Catholic enough in your rhetoric.)
Vatican II opened the door to important dialogue and meaningful new ways for us to go forward. There is simply no place for anti-Catholicism in our time, especially if you have a genuinely historic view of the development of the truth and of God’s church. We who are Protestants didn’t arrive on the scene out of nowhere. When we think this way we act more like Protestant Gnostics than real Christians. It is time we confess that we should be properly catholic and apostolic, rather than sectarians who use the Bible for our own end. Roman Catholics and the Orthodox will still insist, in good conscience, that they are the catholic and apostolic church but this debate is a different matter and must be discussed in a different way. I fear for a Protestant future that continues to promote sectarianism as essential to sola Scriptura.