Paul reminds the Galatians that the message he preached was not received from men, nor was it taught by men, but came by the direct revelation of the risen Lord Jesus Christ. The gospel of the grace of God originated with Christ Himself, and it was Paul's duty, as it is ours, to stand in defense of the gospel.
Paul's own testimony magnifies the power of God's grace. Once a zealous persecutor who trusted in his religious credentials, Paul came to count all of his righteous works as dung. By God's grace, Paul was saved and made an apostle of the Lord and a pattern for all sinners who would be saved in this dispensation of grace.
Our salvation and hope are in Christ alone, proper understanding of the gospel of God's grace is essential for walking in the liberty and purpose He has given us.