Especially important in the argument of the letter (Galatians) is the idea of incorporation into Christ. God promised to bless the nations “in” Abraham (3:8), and that blessing is now given to all who belong to Christ (3:14) because he is the (singular!) “seed” of Abraham (3:16). So, Paul concludes in the climax of his theological argument, “in Christ Jesus you are all children of God through faith” (3:26). Incorporation into Christ is a fundamental theological concept in the letter (as it is in Paul’s theology generally). It is the theological center from which the various lines of Paul’s theological reasoning radiate. Those who are “ in Christ” enjoy the “blessing of Abraham,” in the context of justification (3:14); and “those who belong to Christ Jesus have crucified the flesh with its passions and desires” (5:24). Thus both justification and sanctification are given believers via their union with Christ—the “double gift” that Calvin is especially concerned to emphasize.”
Galatians
by Douglas J. Moo