On Galatians 3:27:
To “put on Christ” may be understood in two ways, according to the Law and according to the Gospel. According to the Law as in Romans 13:14, “Put ye on the Lord Jesus Christ,” which means to follow the example of Christ.
To put on Christ according to the Gospel means to clothe oneself with the righteousness, wisdom, power, life, and Spirit of Christ. By nature we are clad in the garb of Adam. This garb Paul likes to call “the old man.” Before we can become the children of God this old man must be put off, as Paul says, Ephesians 4:29. The garment of Adam must come off like soiled clothes. Of course, it is not as simple as changing one’s clothes. But God makes it simple. He clothes us with the righteousness of Christ by means of Baptism, as the Apostle says in this verse: “As many of you as have been baptized into Christ have put on Christ.” With this change of garments a new birth, a new life stirs in us. New affections toward God spring up in the heart. New determinations affect our will. All this is to put on Christ according to the Gospel. Needless to say, when we have put on the robe of the righteousness of Christ we must not forget to put on also the mantle of the imitation of Christ.