Acts 16:21

Customs which it is not lawful for us to receive, or to observe, being Romans

(εθη α ουκ εστιν ημιν παραδεχεσθα ουδε ποιειν Ρωμαιοις ουσιν). Note the sharp contrast between "being Jews" in verse Acts 16:20 and "being Romans" here. This pose of patriotism is all sound and fury. It is love of money that moves these "masters" far more than zeal for Rome. As Roman citizens in a colony they make full use of all their rights of protest. Judaism was a religio licita in the Roman empire, only they were not allowed to make proselytes of the Romans themselves. No Roman magistrate would pass on abstract theological questions (Acts 18:15), but only if a breach of the peace was made (εκταρασσουσιν ημων την πολιν) or the formation of secret sects and organizations. Evidently both of these last points are involved by the charges of "unlawful customs" by the masters who are silent about their real ground of grievance against Paul and Silas. Εθος (kin to ηθος, 1 Corinthians 15:33) is from εθω, to be accustomed or used to a thing. The Romans granted toleration to conquered nations to follow their religious customs provided they did not try to win the Romans. But the Jews had made great headway to favour (the God-fearers) with increasing hatred also. Emperor worship had in store grave peril for both Jews and Christians. The Romans will care more for this than for the old gods and goddesses. It will combine patriotism and piety.