“But when I saw that they walked not uprightly according to the truth of the gospel…”
No one except Paul had his eyes open at this, and so it was his duty to reprove Peter and his followers for swerving from the truth of the Gospel. Both Paul and Peter need to be commended, however; Paul for his courage and Peter for his humility and repentance. When Paul reproves Peter’s racial pride and cowardice by declaring that he was not living “uprightly according to the truth of the gospel,” he lays down a powerful principle about the centrality of the gospel. The principle is that the Christian life is a process of renewing every dimension of our life by thinking and living out the ramifications of the gospel. In other words, the gospel is to be applied to every area of thinking, feeling, relating, working, and behaving.
The implications and applications of this are many. One implication is that the only way to be changed by the power of God is to bring the gospel truth to bear on every area of life. This is the only way we grow (Gal 3:1-3) and are renewed (Col 1:6). It is the solution to each problem, the key to each closed door, the power through every barrier (Rom.1:16-17).
This is because the gospel is the power of God itself, for Paul says “I am not ashamed of the gospel, for it is the power of God for salvation” (Rom.1:16). It is also the blessing of God with benefits, which accrues for anyone who comes near (I Cor.9:23). It is even called the very light of the glory of God itself—”they cannot see the light of the gospel of the glory of Christ…for God…has made his light shine into our hearts to give us the light of the knowledge of the glory of God in the face of Jesus Christ.” (II Cor.4:4,6)
“The gospel is not a doctrine of the tongue, but of life. It cannot be grasped by reason and memory only, but it is fully understood when it possesses the whole soul and penetrates to the inner recesses of the heart.”
(John Calvin, Golden Booklet of the True Christian Life)