"For I would have you know, brothers, that the gospel that was preached by me is not man's gospel. For I did not receive it from any man, nor was I taught it, but I received it through a revelation of Jesus Christ. For you have heard of my former life in Judaism, how I persecuted the church of God violently and tried to destroy. And I was advancing in Judaism beyond many of my own age among my people, so extremely zealous was I for the traditions of my fathers. But when he who had set me apart before I was born, and who called me by His grace, was pleased to reveal His Son to me, in order that I might preach Him among the Gentiles, I did not immediately consult with anyone, nor did I go up to Jerusalem to those who were apostles before me, but I went away into Arabia, and returned again to Damascus.
Then after three years, I went up to Jerusalem to visit Cephas and remained with him fifteen days. But I saw none of the other apostles except James the Lord's brother. (In what I am writing to you, before God, I do not lie!) Then I went into the regions of Syria and Cilicia. And I was still unknown in person to the churches of Judea that are in Christ. They only were hearing it said, 'He who used to persecute us is now preaching the faith he once tried to destroy.' And they glorified God because of me."
In this passage, Paul is giving his evidence of how he was called by God in order to preach the Gospel. This is because in the previous section he told the Galatians not to believe the false teachers, and he is now showing that he is not false, but that he really was sent by God. He makes sure to tell them that the Gospel that he received is not man's gospel, but God's. He tells the Galatians that he received the Gospel from Jesus Christ Himself, and then he goes on to explain why it had to have been from Christ.
Paul was a persecutor of the Christian church before he converted to Christianity. Because of this, there could be no way other than Christ talking to him Himself that he would make such a drastic transformation.
One thing that's important to point out is how Paul talks about the fact that he was "set apart." In verse 15 he says, "But when He who had set me apart before I was born and who called me by His grace..." Now, this may not be the point of the passage, but Paul is talking essentially about predestination in this case. He does not say that he chose Christ, but that Christ chose him and chose to reveal himself. Paul did nothing on his own to believe in God, but God CHOSE HIM BEFORE HE WAS BORN. This would allude to the belief of the elect. But I digress. That's not the point of the passage.
Continuing on into the passage, Paul goes on to explain where he went after his conversion. In verse 20 he tells the Galatians that he is not lying, which would seem to suggest that he is telling them these things in order to convince them that the things that he has to say really are truth, and he is not out for his own gain.
Then at the end of the passage Paul talks about how other people noticed that he used to persecute the Church, but had changed and was preaching the Gospel. Because they saw him doing this, he says that they glorified God.
This leads into the application part. Throughout this whole passage Paul is giving evidences of and reasons why he believes what he does, and at the end he says how it affected other people. The people GLORIFIED GOD because of him. I believe that it is important for Christians as a whole to make sure that we are giving reasons for our faith and also telling others about who we used to be, so they can see the transformation that has taken place. If they cannot see it, then they cannot glorify God as the Gentiles had done when Paul preached the Gospel.
We need to be willing to be transparent. We need to be willing to share with others our own failings, especially before transformation has taken place. Now in some cases, like mine, we can't remember what our lives were like without Christ in them because we accepted Christ at a young age. If we can't share about our lives before Christ, then, we need to be able to talk about how God has transformed us in our walk with Him. Being transparent does an awesome thing for the Kingdom of God, so this makes it so important not to hide things that have happened in our lives.