Sun-young Kim, Luther on Faith and Love: Christ and the Law in the 1535 Galatians Commentary, Emerging Scholars (Minneapolis: Fortress Press, 2014).199–200.
The logical significance of this metaphor of tree and fruits is the inner necessity of every tree to bear fruits—namely, the external evidence of its being internally alive. In the same way, those who grasp Christ by faith in their hearts are like “trees planted by streams of water, which yield their fruit in its season” (Psalm 1:3). Concentrating on the inner necessity of a tree bearing fruits, Luther articulates: a person who has Christ by faith in his heart requires no external command to work, since he has an internal driving force originating from Christ in his heart. . . . The Spirit of Christ “does not permit a man to be idle but drives him to all the exercises of devotion, to the love of God, to patience in affliction, to prayer, to thanksgiving, and to the practice of love toward all men.” Although fruits do not make a tree but are produced by a tree, they are sure evidence of its being either alive or lifeless. Fruits are external signs of the identity and internal health of a tree. Righteous works likewise distinguish an authentic Christian from a fake and bear witness to the spirit health of a person.