What does Acts 20:22 mean?

When Paul was younger, he trained as a Pharisee under the famous teacher Gamaliel. He learned to follow the Mosaic law faithfully and legalistically. When the church started in Jerusalem, he first witnessed and approved of the murder of Stephen the evangelist, then imprisoned believers in Jerusalem, then followed them to other countries to bring them to trial before the Sanhedrin in Jerusalem (Acts 7:58; 8:1–3; 9:1–2). Now, Paul must tell the elders of the church of Ephesus that he will face the same trials.

He wants to get to Jerusalem by Pentecost (Acts 20:16), but the Holy Spirit has been warning him his stay will not be pleasant. He doesn't know details except he will be imprisoned and afflicted (Acts 20:23), and that God promised this fate long ago (Acts 9:16).

He probably doesn't know the arrest will start with false accusations by Jews who are from the very province—Asia—where they are meeting, that the Jews will believe the false accusation that he brought a Gentile into the temple, or that they will beat him until the Romans come to the rescue by arresting him (Acts 21:27–36). He doesn't know he will barely escape flogging and an assassination plot before facing house arrest in Caesarea Maritima for two years (Acts 22:23–29; 23:12–16, 23–24; 24:27). Or that from Caesarea, he will take a harrowing sea voyage until the ship is wrecked on a tiny island where he will promptly get bitten by a viper (Acts 27:13—28:6).

"Constrained" is from the Greek root word deo which literally means to bind or fasten with chains. Metaphorically, it means to be attached to or under the obligation of something or someone. Paul is as bound by the Holy Spirit's leading as he will be by the chains the Romans will fasten to him (Acts 21:33). The Holy Spirit has chosen what Paul will experience, and he walks into it willingly, despite even the protestations of his friends (Acts 20:37–38; 21:12). His perspective is what ours should be: "What are you doing, weeping and breaking my heart? For I am ready not only to be imprisoned but even to die in Jerusalem for the name of the Lord Jesus" (Acts 21:13).