Quoting Dr. Martin Luther from his lecture on Galatians 1:4, 5:
Therefore since Christ rescues you from the world, He certainly rescues you from yourself as from the worst enemy of all, just as Paul says in Rom. 7:18: “Nothing good dwells with me, that is, in my flesh.” By your own strength, therefore, you will not overcome the evil world and your own vices; your works are in vain unless Christ alone delivers you. So beware lest fastings, vigils, zealous efforts, temperance, sobriety, and other virtues make you an incorrigible hypocrite.
Martin Luther, vol. 27, Luther's Works, Vol. 27 : Lectures on Galatians, 1535, Chapters 5-6; 1519, Chapters 1-6, ed. Jaroslav Jan Pelikan, Hilton C. Oswald and Helmut T. Lehmann, Luther's Works (Saint Louis: Concordia Publishing House, 1999, c1964), 27:174.