Why are we called Lutherans?
It is not we who call ourselves Lutherans. Rather, our adversaries call us that. We allow this to the extent that this title is an indication of the consensus that our churches have with the orthodox and catholic doctrine that Luther set forth from Holy Writ. Therefore we allow ourselves to be named after Luther, not as the inventor of a new faith but as the asserter of the old faith and the cleanser of the church from the stains of Papist dogmas. Consequently, we also do not reject the names “Christian” and “catholic,” nor do we render ourselves unworthy of them by the approval of any heretical dogma, as did the Arians, Nestorians, Eutychians, etc. Rather, we are called “Christians” from Christ as the only Author and Teacher of our faith. We are called “catholics” from our consensus with the catholic faith. We are called “Lutherans” from Luther as the asserter and defender of that faith, but especially as the reformer whom God raised up.
—Johann Gerhard, On the Church (Theological Commonplace XXV), § 156.
HT: Chris, at Lutheran Kantor