What does Acts 16:18 mean?

When Jesus was teaching in a synagogue early in His ministry, a demon that had possessed a man cried out, "I know who you are—the Holy One of God." Jesus' first command was, "Be silent" (Mark 1:21–25). When Jesus was teaching and healing by the Sea of Galilee, demons would declare, "You are the Son of God." Jesus "strictly ordered them not to make him known" (Mark 3:11–12). This was His standard way of dealing with demons: "he would not permit the demons to speak, because they knew him" (Mark 1:34). There is a noticeable exception in the situation with the men at the tombs in the Gadarenes who were possessed by Legion (Matthew 8:28–34). In that case, Jesus spoke with Legion, negotiating a transfer to a herd of pigs (Mark 5:1–13). The difference seems to be that in the Gadarenes, only the disciples witnessed the interaction.

Here, Luke tells the story of a girl possessed by a fortune-telling demon. The spirit has been harassing him, Paul, Silas, and Timothy every day as they walk to the riverside outside Philippi to pray. Through the girl, the demon has announced the men serve God and proclaim how to be saved (Acts 16:16–17). It's not clear why Paul waited to deal with the situation until now. It may be that Jesus silenced the demons only because He didn't want the people to know yet that He was the Son of God. Explaining Jesus' identity is Paul's entire mission. It may be that Paul anticipates freeing the girl will cause problems with the men who own her—which it does (Acts 16:19).

Finally, Paul expels the demon. The girl is freed but no longer able to tell fortunes. This does not please her owners who made quite a bit of money by taking advantage of her condition. They drag Paul and Silas before the town leaders and charge them with teaching about foreign gods (Acts 16:20–21). Although the Roman Empire is open to all sorts of religion, any "gods" must be officially approved by the government. Philippi is a Roman outpost, so the law is enforced more strongly. Although Judaism is a recognized religion, worship of Jesus Christ is rapidly distinguishing itself. Especially problematic for Romans, Christians elevate another man above the deified emperor.

The law is so important to Romans that the people and magistrates beat and jail Paul and Silas. The next morning, the magistrates learn that Paul and Silas are not unsophisticated yokels: they are Roman citizens. It is as great a crime to disrespect Roman citizens as it is Roman gods. When the magistrates learn of their error, they officially apologize and meekly request the two move on (Acts 16:35–40).