“Before concluding his letter Paul returns once more to the antithesis of cross and circumcision, setting them forth this time as representing respectively the true and the false ground of boasting, and thus carrying a stage further his polemic against the Judaizers and their way of legal observance (Galatians 5:2-12).”
Paul ended most, if not all his letters similar to his statement in Galatians 6:11 (1Corinthians 16:21-24; (Colossians 4:18; 2Thessalonians 3:17). It was similar to our signature, saying we are responsible for what comes before. In Paul’s day folks had to beware of letters sent by enemies but claiming they came from the apostles (cf. 2Thessalonians 2:2).
Two things that motivated legalists to preach law instead of Christ was: first, they delighted in making others like they were (cf. Matthew 23:15); and secondly, they did this in order to escape persecution. They made their boast in the Law, and subsequently in their own behavior and in the behavior of others whom they convinced to embrace the Law. Paul, however, completely dismisses both of these motives in Galatians 6:13-14. The legalists don’t keep the law, nor do those in whom they make their boast, i.e. the people they’ve converted. If no one keeps the Law, how can boasting in the Law benefit anyone? The only true boast, according to Paul, is our boast in the finished work of Christ. The cross! Christ, and him crucified is the only thing worth boasting about.
Sometimes born-again believers make this same mistake when we make a big deal over getting others to believe as we do and boast of increasing the numbers of our group. Doctrines never saved a soul—only Jesus saves. Often we boast of the greatness of one denomination over another, but what does that have to do with Christ saving us? Did Jesus found our denomination? Making others as we are does not serve the Gospel of Christ. Causing other to trust in Jesus is what saves. A greater numbers of folks like us doesn’t serve the Gospel. A greater number of folks looking to Jesus serves the Gospel.
If Paul were writing to folks today, instead of circumcision, he might use terms like dispensationaism, evangelical, denominational or independent, or he might speak of liberals and conservatives. While it may be fun to investigate God’s word and arrive at our own conclusions, all this doesn’t matter as for as living out the life of Christ is concerned. What really matters is anyone who is in Christ is a new creature (Galatians 6:15; 2Corinthians 5:17-21).
Folks are either in Christ or in Adam (1Corinthians 15:22, 45). We are either abiding in death (the old creation) or in life (the new creation). So, our only boast can be in Christ and him crucified—all else takes away from the work of Christ and causes folks to look to the flesh to serve God. If God is not looking at the sins of others (2Corinthians 5:19), why do we need to point them out? Our ministry is not to root out sin, but to let folks know what God has done for them. The ministry of the Spirit of God through the preaching of the Gospel will convict the world of sin and not only so, but also of righteousness and judgment (cf. John 16:8). The Spirit does it all. All we need to do is preach the Gospel.
The final blessing of grace Paul prayed in Galatians 6:18 was over the brethren at Galatia, because they had removed themselves from the grace of Christ by embracing the law and its curse. In Galatians 6:16 Paul prayed peace upon the believers in Galatia, and mercy upon the Jews who are under the curse (cf. Galatians 1:8-9) at Jerusalem (cf. Romans 2:27-29), because it was they who were ultimately responsible for the trouble Paul mentioned in Galatians 6:17. It not only involved matters such as what occurred in Galatia but outright persecution (cf. 2Corinthians 11:23-27). Even though the agitators in Galatian thought themselves to be believers in Christ, Paul showed the difference between them and him, in that he paid a difficult price to preach Christ the way he did, but the legalists preached what they did in order to escape such persecution.
 Ronald Y. K. Fung, The Epistle to the Galatians; page 300