Luther on what it means to crucify the flesh, commenting on Galatians 5:24. A real gem, imo.
24. "And those who belong to Christ have crucified the flesh with its passions and sins."
This whole discussion of works shows that true believers are not hypocrites. Therefore no one should deceive himself. Whoever belongs to Christ, says Paul, crucifies the flesh with all its diseases and faults. For because the saints have not yet completely shed their corrupt flesh, they are inclined toward sinning. They do not fear and love God enough, etc. They are aroused to anger, envy, impatience, sexual desire, and similar feelings; nevertheless, they do not carry out these feelings, because, as Paul says here, they crucify their flesh with its passions and faults. This takes place when they not only repress the wantonness of the flesh by fasting or other kinds of discipline, but when, as Paul said earlier (5:16), they walk by the Spirit; that is, when the threat that God will punish sin severely warns them and frightens them away from sinning; and when, instructed by the Word, by faith, and by prayer, they refuse to yield to the desires of the flesh. When they resist the flesh this way, they nail it to the cross with its passions and desires. Thus although the flesh is still alive and in motion, it cannot accomplish what it wishes, because it is fastened to the cross by its hands and feet. As long as they live in this world, therefore, the faithful crucify their flesh; that is, they are aware of its desires, but they do not yield to them. Dressed in the armor of God, with faith, hope, and the sword of the Spirit (Eph. 6:11–17), they fight back at the flesh; and with these nails they fasten it to the cross, so that against its will it is forced to be subject to the Spirit. Eventually, when they die, they will put it off completely; and in the resurrection they will have a flesh that is pure, without any passions or evil desires. [LW 27:96-97]