In Galatians 2 Paul recalls a confrontation with Peter-—and the crucial issue is justification by faith in Christ.
“Even in Galatians, Paul’s teaching on justification has its stark urgency, not simply because church unity is at stake. His rebuke of of Peter’s conduct is so unsparing, not just because unity between Jew and Gentile is being jeopardized, but because of what that broken unity is symptomatic, because he sees that such conduct strikes at ‘the truth of the gospel’ (2:14). Moreover, it conflicts with that gospel truth because the gospel, as he expresses it programmatically elsewhere, is not the reflex, post facto, of having been saved. Rather, it is ‘the power of God unto salvation’ (Rom. 1:16), or even more tersely, ‘the gospel of your salvation’ (Eph. 1:13).” (Richard B. Gaffin, Jr., By Faith, Not By Sight, p. 51).
“A man who tries to earn his salvation, or to do anything towards earning it, has, according to Paul, done despite to the free grace of God.” (J. Gresham Machen, Machen’s Notes on Galatians, p. 143).
From the Reflection for Trinity Presbyterian Church of the OPC