It was not inconsequential for Peter to go from eating with the Gentile Christians at Galatia to avoiding believers who did not keep circumcision, kosher, or other similar parts of the Law (Gal. 2:11–13). Such actions divided Jesus’ followers according to their fidelity to the Mosaic law, thereby removing Christ and His work as the defining mark of the new covenant community. Plainly, this was not Peter’s intent; still, his acts called into question Christ’s sufficiency, and so Paul rebuked him publicly lest others be led astray (v. 14; 1 Tim. 5:19–20). Augustine says Paul did this “that everyone might be bettered by his rebuke” (Ancient Christian Commentary on Scripture, NT vol. 8, p. 37).
The English Standard Version ends Paul’s rebuke at Galatians 2:14, but since the original Greek has no quotation marks, his words to Peter may not end until verse 21. Either way, today’s passage begins a transition from Paul’s defense of his office (1:11–2:14) to his exposition of faith (chap. 3–4). Of central import here is justification — being declared righteous in God’s sight. The Judaizers said we are right before our Creator through faith in Jesus and our obedience to the Law, but for Paul this amounts to our being justified by our own efforts. As John Calvin comments, 2:15–16 shows that “either nothing or all must be ascribed to faith or to works.”
For Peter to imply in his actions that Gentiles must live like Jews to be true Christians was hypocritical because even he, a Jew, did not live like a Jew (v. 14). He actually knew from experience that all who rely on their own works of the Law for justification will fail every time (Acts 15:6–11). If even Paul and Peter, Jews who had the oracles of God (Rom. 3:1–2), could not obey these oracles, how could they rightly lay the burden of the Mosaic law — the yoke of the Torah — upon Gentiles? But Paul does not argue merely from experience, all of Scripture points to the inability of Israel to be declared righteous through the Law (Josh. 24:19; Acts 7:51–53), an inability rooted in sin, not in the Torah itself (Rom. 7:7).
A more detailed look at the works of the Law, the ground of justification, and so on will come in due time. Before we press on, however, we see clearly that we are declared righteous before God through faith in Jesus alone (Gal. 2:15–16).