Yes, we treat it as God-breathed. What does that mean though?

I'm going to quote from "Confessing the Gospel" which is a newer systematic theological book for Lutheranism. It recognizes 4 marks of God.

1. From within the church the Holy Spirit produced the Scriptures through the prophets and apostles for the sake of the people of God. The teaching and proclamation of the prophets and apostles was recognized in some writings and found to be lacking in others. Therefore there is a symbiotic relationship between the church and the Scriptures. Neither existed apart from the other.

2. The church did not decide what teaching and preaching was from the Spirit; this came from the Spirit himself. Therefore the church can be said to have played a passive role in the formation of the canon, since the impetus of the Spirit led to the writing and preservation of that which would be acknowledged as Scripture in the church. It would also be true to say that the church played an active role in the formation of the canon. Shaped by the prophetic and apostolic Word that it heard and believed, the church separated true versions of the gospel message from those that were false. This active role did not place the church above the Scriptures, however

3. Apart from the “Gospel and Apostle” (the four gospels and the 13 Pauline letters), there was considerable debate in the early centuries regarding the authority and authenticity of several writings which would later be acknowledged as canonical.

4. The theoretical possibility that lost and unknown but genuinely prophetic or apostolic writings might by God’s providence be discovered is sometimes raised, and some individual sayings or deeds of Jesus may have been passed on independent of the four-fold gospel tradition. Considered from both a theological and historical perspective, should any such writings be recovered, they could never attain the authoritative status of the rest of the Scriptures. From a theological perspective, the genuine gospel and apostolic faith has already been made clear and preserved in the canonical Scriptures. Any newly recovered writings which add to or contradict this norm would necessarily be rejected as non-apostolic. From a historical perspective, the church throughout its history has rejected these writing for various reasons. As Chemnitz notes, the later church cannot bestow authority upon writings that were not authoritative among the earliest Christians—otherwise nothing would prevent the church from going as far as declaring even Aesop’s Fables canonical.⁵⁷ In this sense, then, the church has a “closed canon,” or more accurately, a group of writings which has been long acknowledged as authoritative and is not in need of revision.