What does Acts 16:21 mean?

Paul, Silas, Luke, and Timothy are in Philippi planting a church. The city apparently does not have enough Jews for a synagogue, so the missionaries meet with God-fearing women by the river. Over the past several days, a possessed girl has been following them, crying "These men are servants of the Most High God, who proclaim to you the way of salvation" (Acts 16:17). Her influence is either distracting or deceptive enough that Paul becomes annoyed and expels the demon from the girl (Acts 16:18).

This causes problems. The demon has given the girl the ability to know things she wouldn't otherwise. She is a slave, and her owners realize she can no longer bring in money through her ability to tell fortunes. Her owners tell the magistrates that Paul and Silas are breaking the law (Acts 16:19–20).

"Advocate customs" means to teach people to worship a god that is not authorized by the Roman government. Cicero, in De Legibus, ii.18 said, "No person shall have any separate gods, or new ones; nor shall he privately worship any strange gods, unless they be publicly allowed." Judaism was legal, but the magistrates might not know too much about it. Jews would quietly worship an "invisible" God, but Paul and his team are talking about worshiping a man from Galilee. In addition, Philippi was a Roman military outpost, so the laws may have been enforced more strongly.

In response, the crowds and the police beat and imprison Paul and Silas. Of course, God works their hardship for good. First, the jailer and his family come to faith in Jesus. Then the magistrates discover their "criminals" are Roman citizens—whom they have illegally harassed. The must officially apologize (Acts 16:22–39). Roman law protected citizens as much as gods.