Romans 1:1c

Paul, a servant of Christ Jesus, called to be an apostle and set apart for the gospel of God—

(v1, NIV)

So Paul has been set apart for the gospel. In what way has he been set apart?

But when God, who set me apart from birth and called me by his grace, was pleased to reveal his Son in me so that I might preach him among the Gentiles, I did not consult any man,

(Galatians 1:15-16, NIV)

God didn't set him apart when he was on his way to persecute Christians. It was since when he was born. When he was in his mother's womb, God set him apart for his gospel. Now, if Paul was set apart since birth, how or why did God allow him to stray as he did? If Paul was set apart for the gospel since birth, how could God let Paul become a persecutor and hater of Christians?

But for that very reason I was shown mercy so that in me, the worst of sinners, Christ Jesus might display his unlimited patience as an example for those who would believe on him and receive eternal life.

(1 Timothy 1:16, NIV)

God allowed Paul to run astray for a season. In doing so, he was able to display his unlimited patience. Now, people can look to God's example in Paul and see the kinds of people God saves. God saves sinners. God even saved the "worst sinner" there is. Name your sins, if you dare. Does any or all of your sins compare to what Paul did? Paul was a murderer of Christians. He hunted them down to bring them to their death, thinking he was doing the work of God. Fine, the point is not to compare. We are all dirty sinners deserving of death. However, before Paul's encounter with the Lord Jesus Christ, we know without a doubt that he was not moving towards salvation by grace through faith in Jesus Christ; he was doing quite the opposite in fact. However, God allowed him to stray and saved in powerfully to show how patient he is. In addition, we know without a doubt that it wasn't any effort of Paul, but a miraculous work of God that saved him and propelled him towards the Gospel of God. God did that. This is God's gospel. It's not about Paul, it's about God.