Holy Scripture (properly defined, ie. the prophetic and apostolic writings of the Old and New Testaments) is the ONLY judge, rule and norm
(richter, regel, richtnur) by which all dogmas, doctrines and teachers are measured in terms of the law and the Gospel. Our Lutheran confessions, as I read them, agree with what scripture speaks and teaches. But they are not on par with Holy Scripture in the standard of measuring apostolic authority.
The interesting question is by what authority the first sentence is true. Is Lutheranism defined first and foremost in terms of justification or sola scriptura?
Not only an interesting question, but a powerfully important one. If Lutheranism is defined first and foremost in terms of sola scriptura, then we are perhaps best understood as a radically reductionistic movement within the church catholic. There are a whole lot of things integral to western Christianity -- liturgy, ecclesiology, ethics, hermeneutics -- that cannot be fully accounted for by scripture alone. For most of the twentieth century, the question of the relation of scripture and tradition (maybe that should be Tradition with a captial "T") was among the hottest topics within ecumenical endeavors; Lutherans from Jaroslav Pelikan to Oscar Cullmann wrote major essays on the authoritative intersection of Bible and Church. Scripture is primary; it is the initial authority; Scripture has priority. But to claim that it is the only authority is to impoverish western Christianity. I hope Lutherans are not interested in that.
Pr. Rahn mentions "apostolic authority." Is "apostolic authority" identical to "scriptural authority"? From a historical perspective, that would seem a very difficult claim to sustain.
There is also a question of solo scriptura refers just to the Bible or to God's Word. I believe that Luther talked more about the Word of God than scripture. As I've noted before, scripture is used to support every Christian heresy. A major shift in the constitutional confession of faith from ALC (which was similar LCMS's) to ELCA is that it does not begin with the Bible.
1. The triune God: Father, Son, and Holy Spirit (and that language is in the Constitution).
2. Jesus Christ as Lord and Savior and the Gospel as the power of God for the salvation of all who believe.
a. Jesus is the Word of God incarnate.
b. The proclamation of God's message to us as Law and Gospel is the Word of God.
c. The canonical Scriptures are the written Word of God.
There are interpretive norms for us to properly hear the Word of God from Scriptures; namely, our confession about the Trinity, our beliefs about Jesus Christ, and the proper understanding of Law/Gospel.