What does Acts 9:5 mean?

On the road approaching Damascus is a zealous Pharisee, Saul. He has orders from the Jewish council to find every Jewish Jesus-follower he can and bring them back to Jerusalem to be tried for the crime of blasphemy. Although Damascus is nowhere near Judea, the high priests have an agreement with the Roman government: they may enforce the Mosaic law among the Jews no matter where they are. Legally, the Sanhedrin cannot execute apostate Jews, but it appears the Romans may look the other way if the situation is right. Stephen was not well known (Acts 7:54–60), while Jesus was popular enough that His death posed the risk of civil unrest.

Saul is surrounded by a great light from heaven and posed with the question of why he is persecuting the speaker (Acts 9:4). He now asks who the speaker is. Jesus told His followers that they will be persecuted because they follow Him, but everything that happens to them will be as if it happened to Him (Matthew 10:17–23, 40; 25:40). As God the Son came in the authority of God the Father, so Jesus' followers come in His authority. Saul is beginning to realize that, while he thought he was defending God's honor, he was attacking God's people (John 16:2). As Jesus told His disciples, "The one who hears you hears me, and the one who rejects you rejects me, and the one who rejects me rejects him who sent me" (Luke 10:16).

This will not be the last time Jesus appears to Saul. On his first trip back to Jerusalem, after preaching that Jesus is the Messiah in the synagogues, Jesus will tell Saul to get out of town and go to the Gentiles (Acts 22:17–21). Jesus will give Saul confidence to continue preaching despite the dangerous situation in Corinth (Acts 18:9–10). And after Saul is arrested, Jesus will tell him he will not die but will teach in Rome (Acts 23:11).