We are “now” justified (Rom.5:1,9). The word “seek” (Gal.2:17) sometimes means to look to something (Amos 5:5,6). All Jews knew where Gilgal was, but it was filled with idols. So God told the Jews not to look to Gilgal to save them from being taken captive by the Assyrians, for Gilgal herself would be taken captive. And Paul was telling Peter they both looked to Christ to be justified, not the law (2:16).
Paul says “God forbid” to the idea that looking to Christ for justification made Him a sinner. He rather says, “I make my-self a transgressor” when I sin (2:17)—specifically if he sinned the sin Peter sinned, rebuilding “the middle wall of partition” between Jews and Gentiles (Eph.2:11-14) that Peter re-built when he stopped eating with Gentiles. Paul’s ministry of grace destroyed the law, the Lord didn’t (Mt.5:17).
So why does Paul say the Lord did (Eph.2:13-15)? Well, that was how the wall was destroyed, not when. Christ’s work on the cross didn’t go into effect until Paul’s ministry, just as November elections don’t go into effect until January. Paul’s telling the Galatians about all this because they too had gone back to the law. They thought the law would make them more holy, but Paul says it makes a man a transgressor!
Romans 7:4 says we are “dead to the law,” and here we learn we are dead to the law “through the law.” Paul calls the law a “ministration of death” (IICor.3:7) because it says sinners must die because they can’t keep the law perfectly (James 2:10,11). But Paul says we died with Christ (Rom.6:3,4).
Of course, once a criminal is executed, the law that condemned him to death can’t condemn him any more. And the law of Moses can’t condemn us any more now that we died with Christ. So now we can “live unto God” (Gal.2:19). The Galatians thought the law would help them live to God, but God gave the law to make sin worse (Rom.7:13), to strengthen sin (ICor.15:56), so unsaved men would know they need a Savior. It wasn’t made for righteous believers (ITim.1:9).
So how are we supposed to “live unto God” (Gal.2:19)? The answer is: under grace, knowing that Christ died for us makes us want to stop living sinfully for ourselves and live unto Him (IICor.5:15). But the Galatians seemed to have forgotten that, so Paul reminds them in Galatians 2:20.
Of course, the only way a crucified man can say “I live” (2:20) is if he rises from the dead—and we were raised with Christ (Eph.2: 4,5). When Paul adds, “yet not I, but Christ liveth in me,” he’s not saying it’s no longer him living, he’s saying it’s both.
This is similar to what happens when a woman becomes “one flesh” with her husband (Gen.2:24). They become one life. In speaking of Adam and Eve, God called “their” name Adam (5:2) because she lost her identity in her husband. She still had her own identity, but now she also had Adam’s. And we still have our old life, but we also have Christ’s.
That’s why Paul says we are “dead to the law…that we should be married to…Him…that we should bring forth fruit unto God” (Rom.7:4). We couldn’t bring God “fruit unto holiness” (6:22) when we were married to the law, but we can now!
But we need the Lord’s help. That’s why Paul says he lived his new life by the faith “of” Christ (Gal.2:20), i.e., His faith-fulness to intercede for us (Rom.8:33,34). Without that, the law could lay plenty of sins to our charge and condemn us!
Clamoring for the law like the Galatians did frustrated God’s grace (Gal.2:21). The word “frustrate” means to defeat (cf. Ezra 4:1,5). Grace can help you live unto God, but the law will defeat God’s grace in that endeavor, because no law can make men righteous (2:21cf.3:21).