Apostle Paul was a devout Jew, a Pharisee of Pharisees, highly learned and knowledgeable of the Law, zealously and blindly guarding Judaism against heresies, but a stiff-necked.
Acts 6:8,10,15 And Stephen, full of faith and power, did great wonders and signs among the people... And they were not able to resist the wisdom and the Spirit by which he spoke... And all who sat in the council, looking steadfastly at him, saw his face as the face of an angel.
All these Jews saw Stephen performing many signs and wonders, preaching the gospel concerning the Messiah with great wisdom, and saw his face like angel (glowed like the face of Moses after meeting God in the tabernacle), yet they condemned him as a heretic.
Acts 7:56, 58b-60 and said, "Look! I see the heavens opened and the Son of Man standing at the right hand of God!"... And the witnesses laid down their clothes at the feet of a young man named Saul. And they stoned Stephen as he was calling on God and saying, "Lord Jesus, receive my spirit." Then he knelt down and cried out with a loud voice, "Lord, do not charge them with this sin." And when he had said this, he fell asleep."
Saul (Paul) also heard that Stephen saw the Lord in heaven. While they stoned Stephen, he still could call on the Lord to receive his spirit and could kneel down to pray for forgiveness as though he was not in pain at all.
Note - Stephen did not taste death so he did not die of pain from stoning.
As promised by God, in peace, joy and comfort and in His presence he slept.
Mat 16:28 "Assuredly, I say to you, there are some standing here who shall not taste death till they see the Son of Man coming in His kingdom."
Brethren, have faith in God. Godly martyrs will not taste the pain of death for our Lord had tasted death on our behalf.
Heb 2:9 But we see Jesus, who was made a little lower than the angels, for the suffering of death crowned with glory and honor, that He, by the grace of God, might taste death for everyone.
Miraculous Salvation of Paul
Acts 9:15-16 But the Lord said to him, "Go, for he is a chosen vessel of Mine to bear *My name before Gentiles, kings, and the children of Israel*. For I will show him how many things he must suffer for My name's sake."
Being a devout Jew, a Pharisee and highly learned, he would not listen to the unlearned fishermen. So God had to save him miraculously on his way to Damascus. God instructed him to reach out not only to Jews only but also to Gentiles and kings
Barnabas seek Paul's Help
Acts 11:22,25 Then news of these things came to the ears of the church in Jerusalem, and they sent out Barnabas to go as far as Antioch... Then Barnabas departed for Tarsus to seek Saul.
After Peter had led Gentiles to the Lord and the elders in Jerusalem agreed that Gentiles were also entitled to the gospel of grace, multitudes of Gentiles were saved.
When the church in Antioch was having revival, Barnabas went to Tarsus for Paul to assist him.
Paul Abandoned the Jews
Acts 18:5-6 When Silas and Timothy had come from Macedonia, Paul was compelled by the Spirit, and testified to the Jews that Jesus is the Christ. But when they opposed him and blasphemed, he shook his garments and said to them, "Your blood be upon your own heads; I am clean. From now on I will go to the Gentiles."
All the Apostles evangelised both Gentiles and Jews. By the time of Paul, most of the devout Jews were already saved. So Paul had to go to the Gentiles.