I am saying this [about Galatians 5:19] in order that you may avoid the wicked errors of the sophists about the holiness of life. Our minds were so obsessed by these errors that we were unable to get rid of them without great effort. Therefore be very careful to distinguish properly between true and hypocritical righteousness or holiness. Then you will be able to look at the kingdom of Christ with eyes other than those that reason uses, that is, with spiritual eyes; and you will be able to assert with certainty that a saint is one who has been baptized and who believes in Christ. Such a saint will also abstain from the desires of the flesh by means of the faith through which he is justified and through which his sins, past and present, are forgiven; but he is not completely cleansed of them. For the desires of the flesh are still against the Spirit. This uncleanness remains in him to keep him humble, so that in his humility the grace and blessing of Christ taste sweet to him. Thus such uncleanness and such remnants of sin are not a hindrance but a great advantage to the godly. For the more aware they are of their weakness and sin, the more they take refuge in Christ, the mercy seat (Romans 3:25). They plead for His assistance, that He may adorn them with His righteousness and make their faith increase by providing the Spirit, by whose guidance they will overcome the desires of the flesh and make them servants rather than masters. Thus a Christian struggles with sin continually, and yet in his struggle he does not surrender but obtains the victory. I have said this to make you understand, not on the basis of human imaginations but of the Word of God, who the genuine saints are. We see that Christian teaching is of the greatest possible help in encouraging consciences, and that it is the sort of teaching that does not deal with cowls, tonsures, rosaries, and similar useless matters but with the most difficult and most important issues, namely, how we are to overcome the flesh, sin, death, and the devil. Because this teaching is unknown to the self-righteous, it is impossible for them either to instruct one erring conscience or to bring comfort and peace to one conscience that is in the throes of terror and despair.
Martin Luther, Lectures on Galatians, Luther’s Works, Volume 27, pages 86-87