Paul Rebukes Peter at Antioch

 But when Cephas came to Antioch, I opposed him to his face because he stood self-condemned, for until certain people came from James, he used to eat with the gentiles. But after they came, he drew back and kept himself separate for fear of the circumcision faction. And the other Jews joined him in this hypocrisy, so that even Barnabas was led astray by their hypocrisy. Gal. 2:11–13 

 But if I build up again the very things that I once tore down, then I demonstrate that I am a transgressor. Gal. 2:18  

What Peter has first pulled down is the wall of partition between Jews and Gentiles. With his yielding to the party of James he is now building this up again and characterising his previous attitude as παράβασις transgression, and himself as παραβάτης transgressor, violator. 

The contrast between “destroy” (καταλύω, katalyō) and “rebuild” (πάλιν οἰκοδομέω, palin oikodomeō) refers most naturally to the law: if Peter, as a Jewish Christian, should try to reinstitute the law as an absolute authority for conduct (as his actions at Antioch suggested he was doing),

 then he would, in effect, be rebuilding that which, in coming to Christ alone for justification, he had earlier torn down

While using different imagery and directed to a slightly different (but related) issue, the language of Eph. 2:14–15 moves in a similar direction: “For he himself is our peace, who has made the two groups one and has destroyed the barrier, the dividing wall of hostility, by setting aside in his flesh the law with its commands and regulations.” The verbs καταλύω and οἰκοδομέω are directly contrasted elsewhere in Scripture only in the Synoptic passages about “tearing down” and “rebuilding” the temple (Matt. 26:61//Mark 14:58; Matt. 27:40//Mark 15:29).

When “I” as a Jewish Christian “rebuild” the law as my authority, I “prove myself to be a transgressor” . The verb συνίστημι (synistēmi) has the sense “to cause something to be known by action” In Peter’s case, his action of withdrawing from meals with Gentiles would be to brand himself, in relationship to his previous eating with them and in many other infringements of torah or its contemporary application, a “transgressor” of that law.

 The lesson would not be lost on the Galatian Gentiles: if they, after coming to Christ for justification, should erect the law as their authority, they, too, would be branded as transgressors

When Paul therefore claims that he has “died to the law,” he means that he has been released from the binding authority of the law of Moses. How foolish, then, for Peter, or any other Jewish Christian, to “rebuild” that authority again (Gal. 2:18)! “The question of transgressing the law does not arise for one who has died in relation to the law” (Bruce 1982b: 142). And how wrong for Jewish Christians, and by extension Gentile Christians such as the Galatians, to try to be justified in terms of the law (Gal. 2: 16).

Douglas J. Moo, Galatians, Baker Exegetical Commentary on the New Testament (Grand Rapids, MI: Baker Academic, 2013), 168–169.

Mention of the incident at Antioch leads Paul on to assert plainly that Jews and Gentiles alike are justified by faith in Christ and not by legal works.

 It is preposterous, he adds, for those who by faith have received the Spirit to go back to reliance on keeping the law.

The gospel proclaims the fulfilment of the promise made by God to Abraham. All the nations were to be blessed with him, and since it was by faith that he received the blessing, so it is with them. The law, far from conveying a blessing, pronounces a curse on the law-breaker. From this curse Christ has redeemed his people by absorbing it in himself through his death by crucifixion. Thus they receive the blessing promised to Abraham—that is, they receive the Spirit through faith.

The superiority of the gospel over the law is shown also in the fact that Abraham received the promise centuries before the law was given. A testamentary disposition, once validated, cannot be invalidated by any subsequent provision; so the promise, confirmed by God, cannot be set aside by the law.

 The law was given not to impart life but to increase the sum-total of sin; the promise, with the righteousness and life which it secures, is obtained not through keeping the law but through faith in Christ.

The law, in fact, is like a slave-attendant placed in charge of a freeborn child until he attains his majority. The people of God have attained their majority through faith in Christ; in him they have entered a new order of existence, in which distinctions of the old order become irrelevant. Until the child comes of age, although he is potentially heir to a rich estate, he is not given his liberty but is treated like a slave. So we remained under the control of the law until the coming of Christ. 

He has redeemed us from legal bondage and given us a new status as mature and responsible sons and daughters of God. It is sons and daughters, not slaves, who receive the Spirit and are enabled by him to call God ‘Abba, Father’, as Jesus himself did. How can one who has come of age desire to be restricted all over again by the leading-strings of infancy?

Paul then appeals to the Galatians to give him the same confidence and affection as they did when first he came to them. They are indeed his dear children, but they have been misled by trouble-makers who simply wish to swell the number of their personal followers.

He next recalls the Genesis story of the expulsion of Hagar and Ishmael in favour of Sarah and Isaac, to show that it is those who enjoy the freedom of the Spirit, not those who are enslaved to the law, that are trueborn children of God. The Galatians have been set free by Christ: let them not turn back the clock and submit to bondage again.

F. F. Bruce, The Epistle to the Galatians: A Commentary on the Greek Text, New International Greek Testament Commentary (Grand Rapids, MI: W.B. Eerdmans Pub. Co., 1982), 135–136.