The authority of the Scriptures is, for Luther, founded on the eternal Word. The eternal Word has taken human nature in Jesus Christ. This is testified of in the Holy Scriptures and in Christian proclamation. The conviction of Luther that only the Scriptures (sola scriptura) are the source of the faith and the norm of Christian doctrine in the context of justification. The Scriptures prove its authority because it is the Word of God manifested in history by which the Holy Spirit arouses the justified in faith. The hub of the Scriptures is Christ, the question is whether they manifest Christ in the Scriptures or not.
In the conflict with the Roman Church Luther substantiates the exclusive authority of the Bible opposite the authority of the ecclesiastical tradition, the pope and the councils. In his polemic with Erasmus, Luther points at the lucidness of the Scriptures (claritas scripturae). The outer lucidness comes about through textual study, the inner lucidness comes about through the Spirit of God. Luther substantiates the necessity of the outer Word for salvation. This was opposite of the Anabaptists, who referred to an immediate spiritual experience. For the reformer the authority of the Scriptures is in the fact that the Scriptures contain words of salvation and life.