In the second half of chapter one, Paul is defending both the gospel he preaches as well as the authority by which he preaches. He reminds the churches in Galatia of who he was before Christ and how he was saved.
When God revealed himself to Paul and commissioned him to proclaim the gospel among the Gentiles, that’s exactly what Paul did. Without consulting anyone else, Paul was obedient to what God called him to do and spent the rest of his life proclaiming that gospel message. As Paul says here, he began by spending some time in Arabia—presumably for a period of spiritual preparation—and then returned to Damascus.

Galatians 1 sets the stage for the letter's overarching defense of the true Gospel. Paul's transformation from a fierce persecutor of the Church to a fervent apostle reinforces the transformative power of God's grace. This chapter serves as a potent reminder to remain grounded in the true Gospel, be wary of distortions, and continuously praise God for His work in the lives of believers.

Paul has been traveling in this second section of Galatians 1. He gives testimony of the changed life he has received, and gives us an abbreviated overview of how he ended up teaching of Christ, rather than persecuting Him. He assures the readers of this letter that this change has come about because of the wonderful works of God. His knowledge of Christ was not taught to him by man, not like he was taught the scriptures of Judaism. Paul says of his new preaching, that it came by “revelation of God!”