In order to establish the authority of the gospel that he preached, Paul found it necessary to tell the Galatians how he came to be a messenger of the gospel of Jesus. His journey was not as the other "apostles" had journeyed; he was late to the party. He underwent a powerful personal conversion, resulting in a 180 degree turnaround of his life, and he spend a number of solitary years quietly seeking to understand the depth of God’s grace.
The Galatian churches knew that Paul was Jewish, but he went out of his way to show them that his preaching and his message were not "products" of the Jerusalem church, or of any human organization. Paul is an apostle precisely because of God's revelation to him, and God's subsequent call to be Jesus' ambassador to the non-jewish world.
Paul's central message is that the progress of the gospel in the world is God's activity. It is not of human origin. Our understanding of salvation may be deepened and perfected as we learn and pray and have fellowship, but salvation isn't achieved through family inheritance, discovered in a library, validated with a diploma, or authenticated by traditions. It comes from God alone.