Charles Ellicott Commentary Acts 16:21

"and set forth customs which it is not lawful for us to receive, or to observe, being Romans." — Acts 16:21 (ASV)

And teach customs.—The word is used to include ritual as well as social habits, and seems to have been specially used in reference to the whole system of Jewish life. (See Notes on Acts 6:14; Acts 15:1; Acts 21:21.)

Being Romans.—The people of Philippi, as a colonia, had a right to claim the title of Roman citizens, which could not have been claimed by those who were merely inhabitants of a Greek city, such as Thessalonica or Corinth. (See Note on Acts 16:12.)