PAUL'S EXPRESSION (vss 11-12)
Paul here is reiterating, or certifying, that the gospel that he preaches is not of man. Its origin is from above, above the world. The gospel we preach is not our gospel but God's gospel. This gospel is not "of," or "according to" (kata) man. Paul did not receive it from (para) man. The gospel was revealed only "by" (di) Jesus Christ.
PAUL'S EXAMPLE (vss 13-24)
This is the crux and the force of Paul's argument. His dedication and certification to the gospel is shown in his change of life. If the gospel does not change a life, then it is not truly in force in a man's life.
Paul's Previous Life, 13-14
Paul's conversation (or way of life) in the past was in the Jews' religion, and that religion persecuted Jesus and the church of the Lord Jesus Christ. Paul went beyond the very religion itself and appended to it persecutions that were "beyond measure," or beyond the measure of the law that was in force against the way of Christ.
This excess shows in three ways: (1) Going "beyond measure," which we have already considered; (2) He profited in the Jews' religion more than any other person had ever done; and (3) Paul was "more exceeding zealous" of the Jews' traditions.
Paul's Changed Life, 15-19
But something happened to Paul: he was immediately changed by God's pleasure, God's separation, God's calling, and God's revelation (see also verse 12). God revealed his Son in Paul (read Acts 9:1-9), that he might preach Jesus among the heathen (Gentiles); and when God revealed Jesus to him, he did not seek out flesh and blood, including the apostles at Jerusalem, but went into Arabia to be taught of the Lord himself. Many believe that he spent three years in Arabia being taught of the Lord (reference is here to verse 18).
Paul's Confirmed Message, 20-24
Because he was taught the message by the Lord himself, Paul's message is not a lie (vs 20). Paul kept himself far away from most men, especially of the apostles; so he was unknown by face to the churches of Judaea. They had only heard of him, but the message was a good one. They had heard that the man who once persecuted the churches is now preaching "the faith," that form of doctrine that is true to Jesus Christ's message to Paul and the other apostles, the same message to both. He preaches the faith that he once destroyed. Then they glorified God in that knowledge. Who, having the true Spirit of God, would not glorify God in the truth of the message of the gospel? Certainly God is glorified every time the gospel is preached; therefore, we should all be witnesses of the one and only true gospel of Jesus Christ, which is revealed, not by men, but by the grace of God. To him be glory forever!