Led by the Holy Spirit, Paul and Barnabas preach their way across the island of Cyprus, Barnabus' home, and make a long stay in Paphos, a port on the west coast of the island. "The proconsul, Sergius Paulus" (v. 7) summons Paul and Barnabas to hear their message, which must have been causing quite a stir.
As Roman citizen, Paul is well-suited to establish a rapport with Sergius Paulus, which he uses to tell him the Good News about Jesus Christ. Paulus, "an intelligent man" (v. 7), is impressed by Paul's message, and is nearly ready to accept faith in Jesus. But then Bar-Jesus, the proconsul's Jewish counsellor and fortune-teller, steps in to interfere.
Filled with the wisdom of the Holy Spirit (v. 9), Paul can see what's going on: Bar-Jesus is operating under an evil influence. Satan (=the force of darkness that opposes God and His works) hates it when people come to believe in Christ—the devil doesn't want us to receive the forgiveness or healing or salvation or eternal life that God wants to give us. Forces that try to turn people away from the winsomness of Christ are finally demonic.
Bar-Jesus means "son of salvation," but Paul perceives that Elymas is really "a child of the devil" (v. 10)—i.e. acting as a proxy for the devil.
Serious problems require serious remedies, just as a brain tumor requires radical treatment. Paul declares the remedy that God will take in this situation: Bar-Jesus will be temporarily blinded. In other words, his inward spiritual blindness will be made clear and concrete in the form of temporary physical blindness.
When Paul declares this: "Immediately mist and darkness came over him" (v. 11=Luke the physician is here using contemporary medical language to describe the scene).
Paul surely recalls his own experience of being blinded on the road to Damascus, which changed his life. God does not cause sickness, although He may allow inward spiritual disorders to take on outward physical form. But God certainly does use times of sickness and physical and mental distress to bring about great change in us, fresh awareness, renewed resolution to live in consonance with His will.
The proconsul is deeply impressed because both the words and the actions of Paul reflect the reality of God. And so, Paul leads Paulus to Christ, his first Gentile convert.