All religions have authoritative texts and persons to whom they look for direction and guidance. This is sometimes called the formal principle of religion. As we have noted recently in our Sunday school class, the formal principle of Orthodoxy is the Bible and sacred tradition. In Catholicism it’s the Bible and tradition and the pope and the magisterium. After the reformation, two traditions—Anglicans and Methodists—would come to emphasize a slightly nuanced position which became known as prima scriptura; the teaching that holy Scripture is first among other places of God’s revelation.
In contrast to all of these, the formal principle of the Protestant reformation was Sola Scriptura: by scripture alone. Alone set this off from Orthodoxy and Catholicism, and later Anglicans and Methodists, and their competing and complementing sources of authority.
Scripture alone is the sole repository of God’s authoritative revelation, the revelation by which all other teaching, traditions, and the like, are to be evaluated and embraced or rejected. This is beautifully summarized in an economy of words in our confessional document, The Westminster Confession of Faith 1.4,
The authority of the Holy Scripture, for which it ought to be believed, and obeyed, dependeth not upon the testimony of any man, or church; but wholly upon God (who is truth itself) the author thereof: and therefore it is to be received, because it is the Word of God.
And again in WCF 1.10,
The supreme judge by which all controversies of religion are to be determined, and all decrees of councils, opinions of ancient writers, doctrines of men, and private spirits, are to be examined, and in whose sentence we are to rest, can be no other but the Holy Spirit speaking in the Scripture.
An additional aspect of Sola Scriptura is the presupposition that Scripture is clear enough to be understood—at least its important and central teachings. Scripture has the quality of perspicuity about it.
The biggest question that Christians wrestle with in this area is: How do we know? How do we know for sure that we are right and they are wrong? It’s an important question. Sola Scriptura suggests that the answer to that question is again to appeal to Scripture itself. Scripture is its own interpreter—the plain making sense of the difficult. Again the Westminster Confession,
All things in Scripture are not alike plain in themselves, nor alike clear unto all: yet those things which are necessary to be known, believed, and observed for salvation, are so clearly propounded, and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them. (WCF 1.7)
The infallible rule of interpretation of Scripture is the Scripture itself: and therefore, when there is a question about the true and full sense of any Scripture (which is not manifold, but one), it must be searched and known by other places that speak more clearly. (WCF 1.9)
The need to think through these issues is alive and well in our day. Sola Scriptura remains the formal principle of the reformation and all churches who descend from that tradition.