Sola Scriptura. Not "Scriptura et magisterium," scripture plus the authority of the church. Not "scriptura et patres," scripture plus the authority of the early church fathers. Not "Scriptura et Aquinas," "Scriptura et Augustine," not "Scriptura et Calvin" (and sorry, I don't know how to make those names properly Latin). Not Scripture plus John MacArthur or John Piper or Mark Driscoll or N.T. Wright or Rob Bell or Greg Boyd either (and I hope I have enough "liberals" and "conservatives" to satisfy the reader that I'm not taking aim at a "side" here). And not "Scripture and my pastor or my bishop or my elders," for these are merely a part of the local incarnation of the Body of Christ, and while we should seek to understand Scripture together in the local body, there is no valid hierarchy or authority among human leaders in biblical interpretation. To the contrary, these and all of the body should have their words evaluated over against Scripture, by all their hearers.
No doctrine or dogma or teaching or credal test dare be claimed with certainty, that is not clearly derivable solely from the properly-exegeted text of the Bible. My choice of the word "derivable" is deliberate. It's not enough to determine that a doctrine is not inconsistent with scripture. It's not even enough that the doctrine, once framed, can be supported by scripture, although in reality I find such claims often fail to withstand careful scrutiny anyhow. I suggest rather that any doctrinal claim should be subjected to the following thought experiment:
Imagine we could find a reader who knew nothing about church history or dogma...one who had never heard of the various heresies and controversies and schisms of the church throughout the century. Imagine further that, though ignorant of the faith, this reader was fluent in Biblical Hebrew and Greek, and was able to read the texts and study them carefully. Would this hypothetical reader be able to come up--solely from studying the biblical texts--with the doctrine at hand? If yes, then we can and should ascribe it serious weight. If no, then however helpful it may be in understanding a difficult passage or concept, it must be considered optional and not core to the faith.
(Even with "core" doctrine, I caution the reader with my previous warning about creeds).
Though it may seem counterintuitive, it is precisely this approach that has led me to dispute the common doctrine of biblical inspiration. Among the areas where I believe scripture must have sole and unchallenged authority, is over the texts' characterization of themselves. So when the text states "thus saith the LORD," we take it seriously as the word of God, but conversely when it says "this is a praise song written by King David," we accept it as a praise song and don't extract doctrine from it any more than we do (or ought) from a hymn by Watts or Wesley, or a chorus by Michael W. Smith or David Crowder.
It's also why I reject credal definitions of the Trinity, eschatology, and many of the other contentious issues that have been used to draw lines and divide people over the stained history of the church. I contend that these dogmas cannot be derived without significant reliance upon extrabiblical authority, and in matters of dogma, there must be no such thing as an extrabiblical authority. Sola Scriptura, taken seriously, leaves one with far fewer certainties and "essentials" than most statements of faith will countenance. And if that makes me another in a long line of church-defined "heretics," well then, I'll just quote Luther again:
"Here I stand. I can do nothing else, so help me God."