The quote comes from Luther's commentary on Galatians, specifically on ch 6, vs 17.
Luther's Works. Volume 27: Lectures on Galatians 1535, Chapters 5-6, Lectures on Galatians 1519, Chapters 1-6
Saint Louis: Concordia Pub. House, 1964. Page 407:
Although “marks”—in Latin this word means signs that are stamped on—may be taken here as referring to the sufferings of Paul, nevertheless—because Paul likes to make use of military allegories and metaphors—he certainly understands them in the sense of the distinctive tokens of the Christian life, which are the crucifixion and subjection of the flesh. In addition, they are the fruits of the spirit. For just as slaves bear the distinctive tokens, the arms, and the colors of their masters, so Paul and every Christian carries in his own body the cross of his lusts and vices—not indeed in the way in which it is customary nowadays to picture on a wall or in paintings and books the distinctive tokens of Christ assembled on a shield. No, every Christian carries this cross in the body—and in my own body, not in someone else’s. What good will it do if you carry even in gold and precious stone, not only the distinctive tokens but also the very nails, yes, the very wounds and blood of Christ, and never express the living image in your body? Moreover, circumcision and the works required by human laws are the marks of Moses and of popes and of Caesars. These alone are looked at now, and they are of such infinite variety that the emperor, together with all his nobles, hardly has so many kinds of distinctive marks."