"Meanwhile," the passage begins; so, I looked back to see what has been happening. Saul had witnessed the execution of Stephen in Jerusalem. Persecution of Christians in Jerusalem became so fearsome that many fled the city. Saul was part of the effort to remove what they considered to be a dangerous threat to their religion. Scaring the Christians out of Jerusalem did not silence them. They preached wherever they were. Evangelism in Samaria was so successful that Peter and John made a trip there then returned to Jerusalem. Philip obeyed a call from the Lord to go south. There he baptized and preached.
Saul, aware of the increase in adherents to Christ, set out to find them and bring them back to Jerusalem.
His journey was interrupted. He saw a light and heard a voice, "Saul, Saul, why do you persecute me?"
He asked "Who are you, Lord?" and was answered, "I am Jesus, whom you are persecuting. But get up and enter the city, and you will be told what to do."
Saul had thought he was persecuting heretics; now, he is struck with the realization that the Jesus that they were claiming had been resurrected was now speaking even to him. And engaging him.
Although we are accustomed to thinking of this event as the conversion of Paul, many commentators prefer to term it as the call. After all, Saul/Paul does not quit being a faithful Jew. The split between Jews who are Christians and who are not will come later.