For it is not sufficient if a man has the Word and the pure doctrine. He must also have the assurance of his call, and whoever enters without this assurance enters only in order to kill and destroy (John 10:10). For God never prospers the work of those who are not called. Even if they teach something good and useful, it does not edify.
--Luther, Martin [Pelikan, Jaroslav Jan (Hrsg.) ; Oswald, Hilton C. (Hrsg.) ; Lehmann, Helmut T. (Hrsg.): Luther's Works, Vol. 26 : Lectures on Galatians, 1535, Chapters 1-4. Saint Louis : Concordia Publishing House, 1999, c1963 (Luther's Works 26), S. 26:vii-20]