"On Preaching" from the Confessions (#17)

Good works should be done because God has commanded them and in order to exercise our faith, to give testimony, and to render thanks. For these reasons good works must necessarily be done. They take place in a flesh that is partly unregenerate and hinders what the Holy Spirit motivates, fouling it with its impurity. Because of faith they are nevertheless holy and divine works, sacrifices, and the reign of Christ, whereby he shows his rule before the world. For in these works he sanctifies hearts and suppresses the devil. And in order to keep the Gospel among men, he visibly pits the witness of the saints against the rule of the devil; in our weakness he displays his strength. The dangers, labors, and sermons of the apostle Paul, Athanasius, Augustine, and other teachers of the church are holy works, true sacrifices acceptable to God, battles by which Christ restrained the devil and drove him away from the believers. (Tappert, The Book of Concord. Fortress Press: Philadelphia, 1959, p.133; Apology IV: 189=90)