The Galatian legalizers wanted them circumcised (v.12) to obligate them to keep the law (cf.5:3), “to make a fair show in the flesh” (v.12)—a fair religious show, like the “shew” of the law (Col.2:20-23), a show Paul calls one of humility. If you get enough people in your religious show, you can make a “fair” show in the flesh. “Fair” in this context means attractive (cf.Gen.6:2). People are attracted to religious shows with lots of people. The legalizers in Galatia did it to avoid persecution (v.12) by unsaved Jews (cf.IThes.2:14). But there’s always a lot of hypocrisy in religion, and Paul says there was among the legalizers too, who didn’t keep the law (Gal.5:13). They were just like the Pharisees whom the Lord lambasted (Mt.23:1-3). Paul did too (Rom.2:17,21,23).
The legalizers also wanted to circumcise the Galatians to “glory” or boast about how many followers they had. Paul says God doesn’t just dislike this, He “forbids” it (Gal.6:14). So Paul gloried in the cross instead, by whom the world was crucified to him. The world didn’t want Christ, so they crucified Him, and once Paul got saved he no longer wanted the world. In the context, he meant the religious world. He called the law worldly in Colossians 2:20,21 as well. We usually think of carnality when we think of worldliness (Tit.2:12), but there’s also a religious kind. It’s crucified to us when we are crucified with Christ (Rom.6:1-6), along with “the flesh” of carnal sins (Gal.5:24) and the “fleshly mind” of religion (Col.2:16-19). The law is called a humility there too, a “voluntary” one, i.e., one God doesn’t ask us for.
Paul adds that he was crucified to the world (Gal.6:14). He was just as dead to the world of religion as it was to him. If you asked the law’s unsaved leaders what they thought about Paul, they would have said, “Paul is dead to us.”
Circumcision doesn’t avail (v.15) or profit (cf.5:2,6) any-thing. It used to, back when Jews had the Scriptures the Gentiles didn’t have (Rom.3:1,2). But now Paul was writing Scripture to Gentiles! And “uncircumcision” didn’t profit either (Gal.6:15), for Paul’s epistles were written to Jews too (Rom.1:5), so Gentiles didn’t have anything they didn’t have. All that matters is the “new creature” God makes you when you get saved (Gal.6:15 cf. IICor.5:17). It was a rule you had to be circumcised to please God, but Paul made a new rule that you don’t (Gal.6: 15), and expects all men to “walk” by it (v.16). He talked about it more when he said he was part of the religious world of the law, but now counted it dung, and asks us to walk by this rule (Phil.3:5-16).
Paul extended “peace” to those who did (Gal.6:16), because the legalizers were troubling them (1:7), and trouble is the opposite of peace (Jer.8:15). He offers them “mercy” because they were being persecuted, and persecution is the opposite of mercy (Ps.109:16). Some say “The Israel of God” (Gal.6:16) is the Body of Christ. They say God cut ties with Israel when they killed His Son and made us “spiritual Israel.” But it’s not spiritual to imply God broke the promise of Jeremiah 31:37. Remember, “they’re not all Israel, which are of Israel” (Rom.9:6). So only saved Jews are true Israel (Rom.2:28,29). That means the Israel of God consisted of leftover kingdom saints. They were still around, circumcising and teaching the law, so Paul extends peace to them so they’d know he wasn’t condemning those things in them, only in the Body of Christ. And he extended them “mercy” for they were still being persecuted too (Acts 8:1).
It troubled Paul that the legalizers were troubling the Galatians (Gal.6:17). They wanted to “mark” the bodies of believers with circumcision, so Paul cited the persecution “marks” he’d incurred in his body trying to prevent that. He calls them the Lord’s marks because He’d be bearing them in His body if he were still here. Finally, the cure to legalism and carnality is grace, so Paul closes this epistle by offering it (6:18) as well as his epistles to the carnal Corinthians (ICor. 16:23).