What does Acts 16:19 mean?

Depending on how a historian defines the term, they might well say that most inhabitants of the Roman Empire were slaves. These ranged from prisoners who worked the mines to indentured servants who sold themselves to government officials to learn an administrative trade. In Philippi, some people own a girl who is possessed by a spirit of divination. They make a great deal of money from her ability to tell fortunes (Acts 16:16). The Bible is silent as to whether demons have any real ability to know the future. Even if demons cannot infallibly predict anything, they have observed humanity for thousands of years. They have more than enough intellect to interpret the actions and words of people around them. Like a con artist with thousands of years of experience and a genius' intellect, a demon could make very good guesses as to what will happen to the girl's clients.

During Jesus' ministry, demons seemed strangely compelled to confront Him (Mark 3:11). One would expect a demon would avoid the One who could destroy them with a thought. The man possessed by Legion in the tombs could have run away; instead, he ran to Jesus (Mark 5:1–7). In a similar way, the girl follows Paul, Silas, Timothy, and Luke for several days, identifying them as "servants of the Most High God, who proclaim to you the way of salvation" (Acts 16:17). Perhaps because he didn't want trouble with her owners, Paul ignores her at first. Her words might be true, but it's not good to let the work of God be associated with a false spirit. And, the girl is probably being obnoxious, interrupting or impeding their ability to preach. Finally, incredibly annoyed, Paul casts the demon out (Acts 16:18).

Usually, Paul's adversaries ran him out of town when traditionally minded Jews believed he was teaching contrary to Scripture. Or, because they were jealous of his popularity (Acts 13:45, 50). This may be the first time Gentiles persecute Paul for financial reasons. It's not the last. During his third missionary journey, a great number of Gentiles will come to faith in Jesus in Ephesus. They will burn their magic books worth 50,000 pieces of silver (Acts 19:19). The local idol makers will take notice. Paul will escape, but the silversmiths, fearful that the worship of Artemis—and the popularity of their idols—will wane, will grab two of Paul's companions and start a riot (Acts 19:23–34).