The story of Paul’s encounter with Jesus on the road to Damascus is recounted several times in Acts: First when the event happens (ch. 9), and then twice retold by Paul, once before Jews and once before Agrippa and Festus (chs. 22, 26).
In the final retelling, Paul quotes Jesus’ words: “I have appeared to you, to appoint you a minister and a witness not only to the things which you have seen, but also to the things in which I will appear to you” (26:16). What do these categories of witness refer to? “What you have seen” refers specifically to the appearance of Jesus near Damascus, which constitutes proof that Jesus was risen from the dead.
But in addition to this dramatic appearance, Jesus promises to appear to Paul in the future. When did that happen? Verse 17 explains: “delivering you from the Jewish people and from the Gentiles, to whom I am sending you.” Every escape from Jewish or Gentile enemies was another appearance of Jesus, and became another saving event to which Paul was a witness.
Paul then had to suffer persecution in order to have something to preach about. He had to be afflicted so he could be a “minister and a witness” to Jesus’ repeated advents, so he could testify that God had fulfilled the promise to the fathers that we, being delivered from fear of our enemies, might serve him without fear. Like David, if he were not attacked, he wouldn’t have anything to sing about.