Jamieson, Fausset and Brown on Acts 16:21

21. And teach customs, which are not lawful for us to receive, neither to observe, being Romans--Here also there was a measure of truth; as the introduction of new gods was forbidden by the laws, and this might be thought to apply to any change of religion. But the whole charge was pure hypocrisy; for as these men would have let the missionaries preach what religion they pleased if they had not dried up the source of their gains, so they conceal the real cause of their rage under color of a zeal for religion, and law, and good order: so Ac 17:6, 7; 19:25, 27.