13“Therefore gird up your minds, be sober, set your hope fully upon the grace that is coming to you at the revelation of Jesus Christ.”
Gird up your minds.
Here Peter is speaking about a spiritual girding of the mind, just as one girds a sword physically to one’s loins. Christ also touched on girding when He said: “Let your loins be girded” (Luke 12:35). In several places in Scripture the loins denote physical unchastity. But here St. Peter is speaking of spiritual loins. In a physical sense Scripture calls the loins the source of the natural descent from the father. Thus we read in Gen. 49:10 that Christ is to come from the loins of Judah. Thus the physical girding of the loins is nothing else than chastity, as Is. 11:5 states: “Righteousness shall be the girdle of His waist, and faithfulness the girdle of His loins.” That is, one suppresses and overcomes evil lust only through faith.
But the spiritual girding—of which the apostle is speaking here—takes place as follows: Just as a virgin is physically pure and blameless, so the soul is spiritually blameless because of faith, through which it becomes the bride of Christ. But if it falls from faith into false doctrine, it must go to ruin. For this reason Scripture consistently calls idolatry and unbelief adultery and whoring, that is, if the soul clings to the teachings of men and thus surrenders faith and Christ. St. Peter forbids this here when he tells us to gird the loins of the mind. It is as if he were saying: You have now heard the Gospel and have come to faith. Therefore see to it that you remain in faith and not be moved by false doctrine, that you do not waver and run hither and thither with works.
Here St. Peter adopts a peculiar expression—different from that of St. Paul—when he speaks of “the loins of your mind.” He uses the word “mind” for what we mean when we speak of being minded, as if I said: “I regard this as right,” or, as St. Paul expresses himself (Rom. 3:28), “We hold that”; that is, “So we are minded.” With this he actually means faith and wants to say: “You have come to the proper understanding, namely, that one is justified by faith alone. Now cling to this understanding. Gird it well. Hold fast to it, and do not let anyone wrest it from you. Then all is well with you. For many false teachers will appear and will set up human doctrines, to take away your understanding and loosen the girdle of faith. Therefore be warned, and give careful thought to this.” The hypocrites, who rely on their works and lead a fine moral life, think that God must take them to heaven because of their works. They become puffed up and arrogant. Like the Pharisee in Luke 18:11–12, they insist on their understanding and opinion. Mary speaks about them in the Magnificat, where she uses the same little word Peter employs here. She says: “He has scattered the proud in the imagination of their hearts” (Luke 1:51), that is, in their understanding.
Sobriety serves the body externally and is the chief work of faith. For even though man has become righteous, he is not yet completely rid of evil lusts. To be sure, faith has begun to subdue the flesh; but the flesh continues to bestir itself and rages nevertheless in all sorts of lusts that would like to assert themselves again and do what they want. Therefore the spirit must busy itself daily to tame the flesh and to bring it into subjection, must wrestle with it incessantly, and must take care that it does not repel faith. Therefore those who say that they have faith, think that this is enough, and, in addition, live as they please, are deceiving themselves. Where faith is genuine, it must attack the body and hold it in check, lest the body do what it pleases.
Luther, M. (1999). Luther’s works, vol. 30: The Catholic Epistles. (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds.) (Vol. 30, pp. 25–27). Saint Louis: Concordia Publishing House.