What does Acts 22:20 mean?

Paul continues an argument which covers three different timeframes. He is in Jerusalem where the temple courtyard meets the Roman outpost, trying to explain to a homicidal crowd why he had walked through the city with a Gentile (Acts 21:27–40).

His defense includes the story of how he came to follow Jesus on the road to Damascus decades before (Acts 9:1–19). After several years learning more about Jesus and evangelizing in and around Damascus, Paul returned to Jerusalem (Acts 9:26–30; Galatians 1:15–24). During his time there, he prayed in the temple and Jesus told him to leave the city. Jesus said the Jews would not listen to him; they'd rather kill him (Acts 22:17–18).

Paul argued that he had hunted and beaten Jesus-followers (Acts 22:19). In fact, he had been present at the murder of Stephen, the first martyr (Acts 7:54–60). Surely his history leant credibility to his current identity as Jesus' follower.

Paul's change of heart meant nothing to the Jews except that he was apostate. Jesus reaffirmed His command for Paul to leave Jerusalem and take the message of reconciliation with God to the Gentiles (Acts 22:21).

The mob doesn't let Paul finish his story. The instant he mentions preaching to Gentiles, they renew their rage and demand the Roman tribune arrest him. The young officer has no idea what is going on but is inclined to acquiesce to their request (Acts 22:22–24).