The following article is quoted in full from the Christian Cyclopedia provided by The LCMS.
Lutheran. (adjective and noun) derived from the name Martin Luther; refers to his followers and to doctrines and practices of the Lutheran Church; apparently 1st used circa 1519 or in the early 1520s, but it may be impossible to determine exactly when and by whom (but first, apparently, by Luther’s enemies). William Warham (circa 1450–1532; Archbishop Canterbury 1504) used the term of followers of Luther 1521. Luther wrote 1522: “True, by any consideration of body or soul you should never say: I am Lutheran, or Papist. For neither of them died for you, or is your master… But if you are convinced that Luther’s teaching is in accord with the Gospel and that the pope’s is not, then you should not discard Luther so completely… It is on account of the teaching that they attack you and ask you whether you are Lutheran” (WA 10 II, 40). “The saving doctrine, the precious, holy Gospel, they call Lutheran.” (Ap XV 44)
Edited by: Erwin L. Lueker, Luther Poellot, Paul Jackson © 2000 Concordia Publishing House.