Lutheranism's formal principle is sola Scriptura: Scripture alone is the supreme and sufficient norm for Christian doctrine and life. The Book of Concord states in its preface that Scripture is 'the only judge, rule, and norm according to which all doctrines should be judged.' Confessions, catechisms, and creeds are authoritative only insofar as they accurately reflect biblical teaching. No church council, pope, tradition, or contemporary prophet can add to or override the authority of Scripture.

Luther polemicized vigorously against the 'Schwärmer' (spiritual enthusiasts) of his day who claimed direct revelations from the Spirit apart from the external Word. The Smalcald Articles and other documents make clear that the Spirit binds himself to the external Word and does not operate through private inner voices independently of Scripture. This does not deny that the Spirit illumines readers of Scripture, but it denies that any new propositional revelation is being given to the church after the close of the apostolic age.