Twice in Galatians 2 Paul speaks of “the truth of the gospel.” In both cases the Apostle had been forced to speak out to defend the purity of “the gospel of the grace of God” (Acts 20:24).
In Gal. 2:4,5 he refers to his contest with those at Jerusalem who would have brought the Gentile believers under the law of Moses. Among them were “false brethren,” he says, “unawares brought in… to spy out our liberty which we have in Christ Jesus, that they might bring us into bondage: to whom we gave place by subjection, no, not for an hour, that the truth of the gospel might continue with you.”
In the second case he refers to a controversy with Peter who, having enjoyed blessed fellowship with Gentile Christians, had been intimidated by some of his Jewish brethren into separating himself from the Gentiles. Concerning this, Paul writes: “But when Peter was come to Antioch, I withstood him to the face, because he was to be blamed” (Gal. 2:11). Why was Peter to be blamed? Gal. 2:14 answers: Because he “walked not uprightly according to the truth of the gospel,” i.e., “the gospel of the grace of God,” in which believing Jews and Gentiles are “one body in Christ.”
How we should all thank God for Paul’s vigorous defense of the gospel of grace, under which all who trust in Christ as Savior are baptized by the Holy Spirit into the one true Bible Church (I Cor. 12:13).
Doubtless Paul’s stand for “the gospel of the grace of God” stemmed from the fact that he himself had experienced the truth of this blessed message. As the chief of sinners he had been gloriously saved. All his power and prestige as a Pharisee, all his intellectual achievement, all his rigid Law observance meant nothing now, as in the presence of the glorified Lord he saw himself a sinner, the chief of sinners, and was saved by the matchless grace of God. source