I. The Church Postil, which Luther himself considered “The best of all his books,” was called forth by the exigency and need of the Church at the time. The majority of the preachers in those days were incapable of working out their own sermons, and were satisfied in reading the Epistle and Gospel lessons, and perhaps besides they read a sermon of another preacher to the congregation. The sermons for this purpose were those by Tauler (d. 1361) and those by Geiler of Kaisersberg (d. 1510).
But since the latter were not in all parts evangelical Luther concluded he would himself write an explanation of the pericopes of the Church year and place the same in the hands of the preachers for their use.
This Luther did not only because the preachers were so incompetent, but also in order to prevent the work of the fanatics and the sects, never however in order to encourage preachers in their laziness to take their sermons from his and other good books, and then never pray, never study and never read and search the Scriptures.
Volume 1, Luther's Sermons, Lenker edition.