Theologians distinguish two principles at the heart of the Reformation: a material principle, justification by faith, which answers what must be believed for salvation, and a formal principle, sola scriptura, which answers where that belief is authorized. The formal principle holds that scripture is the sole infallible source and final judge of Christian doctrine, sufficient in itself to establish what must be believed and practiced, standing above popes, councils and even the church's own confessions when the two conflict. It was not a claim that no other writing carries weight; Luther read the church fathers closely and never treated the Bible as the only book worth consulting. The claim was narrower and sharper: only scripture binds the conscience with final authority, because only scripture is the Word of God in the sense that guarantees it cannot err.
The principle emerged from conflict rather than abstract theory. Pressed at the 1519 Leipzig Debate to say whether a council could err, Luther eventually answered yes, breaking with an assumption nearly everyone in Western Christendom still held. Two years later at the Diet of Worms he refused to recant unless refuted by scripture or plain reason, since, he said, popes and councils had contradicted each other and even themselves. The Formula of Concord later made the principle a formal confessional statement, opening by naming scripture the only rule and norm by which all doctrines and teachers alike are to be judged, with the church's own creeds and confessions ranked as a subordinate testimony to what scripture already teaches, not a second source alongside it.