Roman Catholic theologians have traditionally attributed doctrinal differences among Protestants to Scripture’s lack of clarity, claiming that doctrinal unity cannot be achieved without an authoritative interpreter of Scripture. Denying that such an interpreter exists in an ecclesiastical hierarchy, Lutherans have maintained that although many passages of Scripture are unclear in themselves, many of them are interpreted according to the rule of faith, the set of perfectly clear statements in Scripture that teach all truly catholic doctrine; this is what the term perspicuity of Scripture means. Rather than placing interpretive authority in human traditions or, at the opposite extreme, ignoring or despising the writings of earlier Christians, Luther and his followers rejoiced in the extent to which those writings agreed with the clear words of Scripture. Without explicitly denying the perspicuity of Scripture, Keith Mathison, in his Shape of Sola Scriptura, advocated a middle course in which the writings of early but post-apostolic Christians have a real authority alongside and yet subordinate to that of the Scriptures. His arguments have influenced not only his fellow Calvinists, but even some Lutherans, in spite of the stand of Luther, the Augsburg Confession, and the Formula of Concord…

The rest of this essay is available from DawningRealm.org (PDF). See also Pieper’s Christian Dogmatics, Volume 1, “Holy Scripture,” Chapters 10 and 14-16, especially pages 361-362 on “the rule of faith.”