The means by which the old man is mortified and the new man is strengthened is the Word of God; properly speaking, however, not the Law, but the Gospel. The Law indeed reveals sin, Rom. 3, 20, but it cannot free the sinner from the curse and dominion of sin, Rom. 7, 5-13. However, also in the process of sanctification the Law must be diligently used by the believer as a means to prepare the way for the Gospel in showing the exceeding sinfulness of sin, as also to point out what works are truly pleasing to God, 1 Cor. 6, 1-10. Thus the believer uses the Law as a mirror (Rom. 3, 20) that ''he may be led to the knowledge of his sins"; as a curb (1 Cor. 9, 27) "to restrain his intractable carnal heart as though by certain bars"; and as a "fixed rule (Ps. 119, 9) according to which he regulates and directs his whole life."
The Formula of Concord aptly declares (Epit., VI, 4): "For although they (the believers) are regenerate and renewed in the spirit of their mind, yet in the present life this regeneration and renewal is not complete, but only begun, and believers are, by the spirit of their mind, in a constant struggle against the flesh, that is, against the corrupt nature and disposition which cleaves to us unto death. On account of this Old Adam, which still inheres in the understanding, the will, and all the powers of man, it is needful that the Law of the Lord always shine before them (mirror) in order that they may not from human devotion institute wanton and self-elected cults (rule); likewise, that the Old Adam also may not employ his own will, but may be subdued against his will, not only by the admonition and threatening of the Law, but also by punishments and blows (curb), so that he may follow and surrender himself captive to the Spirit, 1 Cor. 9, 27; Rom. 6, 12; Gal. 6,14; Ps.119,1 ff.; Heb.13,21 (Heb.12,1)."
While the Law thus reveals sin (mirror), restrains the flesh outwardly (curb), and guides the Christian to good works (rule), the power to accomplish sanctification and to do good works comes alone from the Gospel, Rom. 12, 1; 1 John 4, 10. 11. According to Scripture it is the Gospel which inscribes the Law in the heart and enables the believer to keep it, Jer. 31, 31 f. While it is true that through the Law externally good works (iustitia civilis) may be produced in men since they are made to fear God's wrath and punishments, it is alone the Gospel that produces spiritually good works (iustitia spiritualis), or works which flow from true faith in Christ and true love to God. Cp. Luther, St. L., XII, 318 ff.
With respect to the chastisements with which God visits His saints on earth, such as poverty, sickness, sorrow, etc., we may say that, while these do not in themselves sanctify the believers, yet they are the means by which God induces them to meditate on His Word, so that through the study of the Law they learn that they indeed deserve the chastening trials of God, and through the study of the Gospel they again comfort themselves with the abiding love of their heavenly Father, Rom. 8, 35-39. Also the blessings with which God endows His saints on earth should lead them to repentance, Rom. 2, 4, namely, by moving them to search the Holy Scriptures, in which the glory of divine grace shines forth in the face of Christ Jesus and from which they constantly draw strength for greater faith and holier service, Ps. 119, 9-16. 105-112.