This chapter offers a comprehensive presentation of the three dimensions of Luther’s Trinitarian doctrine of grace. (1) The conversion of the sinner and the birth of faith in Christ, justification “through faith alone,” is effected by prevenient grace, the sole work of God’s Spirit. (2) Participation in (2a) the cross and resurrection of Christ as well as in his (2b) person, life, and divine properties, are possible solely because of the presence of the Holy Spirit in the believer. Justification means simultaneously (2a) the forensic declaration of the guilty non-guilty on the basis of the atonement by Jesus’ cross (favor), as well as (2b) a union with Christ in the Holy Spirit (donum). The believer participates both in the person and life of the incarnated Son of God and in the historical facts of salvation in the life, death, and resurrection of Jesus Christ. (3) Sanctification means the gradual growth of love for God and neighbor enabled by participation in divine love in the Holy Spirit who also enables the believer to cooperate with grace. Luther’s dependence on Augustine’s doctrine of grace is pointed out. The three-dimensional structure of Trinitarian grace offers an advancement to the Finnish school of Luther interpretation initiated by Tuomo Mannermaa. His fundamental finding of the participatory nature of justification, rooted in Patristic soteriology, is verified in the present study, but an amendment is also offered, based on a critical analysis of Mannermaa’s interpretation of Luther’s Lectures on Galatians (1531/1535).