As Paul continues the defense of his apostleship begun in verse 11, he notes that the first time he met any of the other apostles was three years after he began his ministry. Further, that on that occasion, Peter was the only apostle he met, during a two week trip to Jerusalem. (vs. 18-19).
Why would he point this out? Because of the Judaizing teachers who claimed that he was usurping his position as an apostle. Perhaps they claimed that Paul was just taking what he had heard, and parroting it while claiming an apostleship he did not deserve.
Paul’s here responds by relating his early ministry. He could not have been parroting the apostles because he hadn’t met or heard them. Instead, as he affirmed in verse 12, “For I neither received it from man, nor was I taught it, but it came through the revelation of Jesus Christ.”
Since the gospel he preached was revealed to him by Jesus Christ, it was authoritative, and to be obeyed by the Galatians (cf. 1:6-9).
