13 For you have heard of my previous way of life in Judaism, how intensely I persecuted the church of God and tried to destroy it. 14 I was advancing in Judaism beyond many of my own age among my people and was extremely zealous for the traditions of my fathers. 15 But when God, who set me apart from birth and called me by his grace, was pleased 16 to reveal his Son in me so that I might preach him among the Gentiles, my immediate response was not to consult any human being. 17 I did not go up to Jerusalem to see those who were apostles before I was, but I went into Arabia. Later I returned to Damascus. (TNIV)In this section, Paul continues to set the record straight. It seems that he had never told the Galatians about his calling to apostleship. The Teachers had, though, using their version of Paul's story to undermine the gospel that he preached. This necessitated that Paul correct their understanding of his past. The second half of verse 13 and verse 14 tell of Paul's life in Judaism prior to coming to faith in Christ. Many commentators point out that it is very possible that Paul was inspired by the story of Phineas in Numbers 25, believing that these followers of Jesus were polluting Israel by not strictly observing the Mosaic Law.
In verses 15 and 16 Paul tells of how his transformation from persecutor to promoter took place. It happened through two means. First God called him from before his birth. Here Paul is using language that echos Jeremiah 1:5, stressing the special nature of his call by God and putting himself on the same plane of authority as Jeremiah (Dunn p. 63). The Teachers were trying to undermine Paul's authority but Paul would have nothing of it.
The other agent of transformation was the revelation of Jesus Christ to him (here I am following the NRSV, Martyn, and Hays) on the Damascus Road. He had an encounter with Jesus Christ that changed him forever. The important point for Paul's argument, though, was that it was with the resurrected Jesus that he received the gospel, not through human preaching (contra The Teachers).
Paul's calling had a purpose; he was called to proclaim the gospel to the Gentiles. This happened at day one. Perhaps the Teachers had told the Galatians about Paul's commissioning in Acts 13:1-3 and claimed that that was when Paul originally received his commission to preach to the Gentiles, and that he was sent under the jurisdiction of the church at Antioch. Paul again rebuts that claim denying any human origin for his calling. His call came much earlier and it was from God through Jesus.
It's interesting to notice that Paul's focus at the end of this section is on what he didn't do. The only explanation is that he is correcting inaccurate stories that The Teachers were telling about him. He was called and set apart by God, and received his gospel directly through a revelation of Christ. No human on earth played a roll in Paul's formation of the gospel. This leaves one open to wonder then, does this emphasis imply that Paul believes that he's the only one who got the gospel right? We'll look at that question when we get to the next section of Galatians.