This epistle was one of Paul's first letters, if not the first. As such, it was early on in his ministry, and one of the first topics in this letter was his credentials. This is seen in his parenthetical statement that he was an apostle appointed by God and Jesus Christ and NOT by men. The other apostles ate, walked, and lived with Christ while He was on the earth, and thus their credibility was established in part by proximity and association with Christ. Paul did not fall into this category, but rather persecuted the true Christians until his dramatic conversion. However, at that time, he was instructed by Christ personally and thus had a legitimate claim to apostleship.
David C. Grabbe
The bulk of this chapter is occupied with Paul's defense of his apostolic office. False teachers,the people who had access to the Galatian Christians' ears,were teaching them that what Paul had previously taught them had no authority from God because Paul did not meet the qualifications of being an apostle.
These people could come up with all kinds of things. They might say, "Well, Paul never met Jesus": that he had not been an eyewitness to Jesus' preaching, that he had received no commission from God to be an apostle, and that he had not even been chosen like Matthias. Paul's calling, conversion, and commission were done apart from large numbers of people. Nobody had seen him trailing around after Jesus as they had seen the Twelve. He had not been eyewitness to the miracles that Jesus did. "He had not been taught directly by Jesus," was what they were saying.
Thus, Paul spends the first chapter and more defending his position. Immediately, he states that his authority did not come through men. He confirms that he was an apostle, but his selection was not of men but by Jesus Christ. Right off he states his authority, and that it had come directly from God. By doing this, Paul puts himself in the same class as the Twelve, because even these false teachers were willing to concede that the Twelve's offices did not come through men either. Everybody knew that they were directly chosen by Christ. So Paul asserts, "So was I."
He also speaks of his experience on the Damascus Road as his commission, and then he references the resurrection, further linking his commissioning to the risen and glorified Christ. All of this is contained within the first three verses. He had to establish his authority quickly, and this is how he chose to do it.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 24)
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