Sola Scriptura is the Reformation view on this question: Where do we go to get God's Word? Christians might all agree that God is the highest authority and therefore His Word is the ultimate standard for all of life, but we disagree on how His Word is communicated to us.
This is essentially my understanding of the spectrum:
|How does God speak to humanity today?|
|Trifold authority: Scripture, tradition, and the Magisterium; but, primarily the pope, being the highest authority of the Magisterium.||Only the 66 books of the Christian Bible. We should no longer expect ongoing revelation now that God has finally spoken in His Son (Heb. 1:2).||Scripture, plus the Holy Spirit speaks directly to each believer, or perhaps directly to various charismatic leaders.|
On Rome's side, the pope is viewed as the successor to the Apostle Peter, and so the pope's official pronouncements are the very words of God. In this way, the speaking and revealing work of the Holy Spirit did not end at the biblical Apostles but continues in the teaching offices of the Roman Church.
On the Anabaptist side, reacting radically to Rome's view, the Holy Spirit could and does presently speak to any Christian, not just the pope or official teachers. Yet, the great similarity this has with Rome is, again, that the speaking and revealing work of the Holy Spirit did not end at the biblical Apostles but continues for those who are sufficiently "tuned in."
The Reformation view, in contrast to both extremes, said that the Holy Spirit was done revealing with the close of the NT canon. The Holy Spirit continues to speak today, but only mediated through Scripture4. And, in my understanding, the Holy Spirit doesn't hijack the words on the page and make them mean something special to each reader; rather, the Holy Spirit meant something when He first inspired the writing, and He means for them to say the same thing today. So God speaks today in the contextual meaning of the words of the Bible.
It is interesting (and concerning) to me how some who would otherwise identify as reformational are reinterpreting passages that provide the basis for Sola Scriptura to support a continuationist view of prophecy, for example. The apostolic age was a unique period in redemptive history, a period which is not today. Lack of appreciation for this fact has most of evangelicalism today looking more like Anabaptists than Reformers. Similarly, Sola Scriptura is commonly misunderstood today to justify a view of "me, God/the Holy Spirit, and the Bible," which is radically individualistic and unappreciative of the rich history of traditions, creeds, and confessions of the church. Reading scripture in isolation from the church (both present-day and historical) actually elevates the authority of the individual self over the authority of God in the scripture.
Related concepts to Sola Scriptura are the perspicuity (clarity) of Scripture and the sufficiency of Scripture. Perspicuity goes to the issue of how to interpret the Bible. Rome said an official teaching office was necessary to interpret Scripture for everyone else, since the Bible is so confusing and all. But the Reformers argued that Scripture isn't that hard to understand, at least on the main points it addresses. So interpretation is performed by all people together, as Horton explains:
In interpreting it, the whole church must be included, including the laity, and they must be guided by the teachers in the church. Those teachers, though not infallible, should have considerable interpretive authority. The creeds were binding and the newly reformed Protestant communions quickly drafted confessions of faith that received the assent of the whole church, not merely the teachers.Anabaptists get around interpretation by allowing any number of meanings, so long as it is what the Holy Spirit told you. But here the Reformers also argued for a literal meaning of the text which we come to by basic principles of hermeneutics and grammatico-historical method. As Sproul says, "Though a text may have a multitude of applications, it has only one correct meaning." Of the Holy Spirit card, Calvin said, "When the fanatics boast extravagantly of the Spirit, the tendency is always to bury the Word of God so they may make room for their own falsehoods."
The sufficiency of Scripture goes to the conviction that everything we need to know regarding religious life5 is in Scripture, and to add anything else is to reduce Scripture's authority. The medieval Roman Church's official theology "was shaped more by the insights of Plato and Aristotle than by Scripture." On the other side, much of evangelicalism today following an Anabaptist stream takes more cues in its message and methods from contemporary politics, sociology, and marketing than from Scripture. In saying "Sola Scriptura," the reformers were rejecting any such influences from Christian theology or practice.
1 "The Crisis of Evangelical Christianity: Reformation Essentials" by Michael S. Horton. Issue: "The Reformation Then and Now" March/April 1994 Vol. 3 No. 2 Page number(s): 12-19
2 I am deliberately avoiding the word "reformed." The Solas in particular trace back to Luther, so "Reformation" to me is inclusive of both Lutheran and Reformed (Calvinist, Presbyterian) traditions. Besides, "reformed" doesn't mean the same thing to everyone anymore.
3 I am aware that some don't think bunching people into a category like Anabaptist is fair, since some Anabaptists were generally orthodox and others were crazies. But as I understand, most of them were crazies, so there.
4 I like how the Westminster Confession of Faith says with present tense, "the Holy Spirit speaking in the Scripture." WCF chapter 1 article 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."
5 By "religious life," I am intentionally qualifying the sphere in which Scripture is sufficient. Westminster Shorter Catechism question 3 is helpful: "Q. What do the Scriptures principally teach? A. The Scriptures principally teach, what man is to believe concerning God, and what duty God requires of man." The sufficiency of Scripture is not at odds with exercising practical wisdom, nor with learning from unbelievers about issues in a common grace realm. As Calvin said in his commentary on the creation in Genesis, "nothing is here treated of but the visible form of the Word. He who would learn astronomy, and other recondite arts, let him go elsewhere." Or as I paraphrase him: if you want to learn astronomy, ask an astronomer (not the Bible).