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 it. 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.
Paul continues to defend himself by saying that he did not even meet with the "apostles before him" (e.g., Peter) after his conversion. He is also an apostle though his apostleship comes not by human agency but from the Lord himself. We are not sure why he went to Arabia but it does show that he didn't depend on the apostles in Jerusalem. As we will observe in the coming chapters, he spent a considerable amount of time away from Jerusalem in the beginning stages of his mission; and yet, he preached the same grace that Peter and the others did. Having established this, to reject what Paul preached to the Galatians is to reject the Lord.
I think this passage outlines what it means to be "called" by God. This portion of Paul's epistle paints a picture of his former life "in Judaism", encounter with Jesus, and his life post-conversion. Paul was called by grace and not because he had great intellectual capacity or mastery of the Law. He was set apart for a purpose and not even his history of persecution could thwart this.
The fact that I believe in Jesus today is a miracle. It brings me great comfort that I had been set apart by God from the beginning for his purpose, and as he did with Paul, he is pleased to reveal his Son to me in order that I may preach him to others. I was called from darkness into light, I have switched kingdoms, I have been brought from death to life. But this is not just for my own comfort, although salvation brings with it a wide range of blessings (Ephesians 1). This is not to be an internal and merely personal thing because believers are called so that they may be sent. And perhaps we will have our own "Arabia" where we receive what God specifically wants us to do.