Albert Barnes Commentary Acts 19:34

"But when they perceived that he was a Jew, all with one voice about the space of two hours cried out, Great is Diana of the Ephesians." — Acts 19:34 (ASV)

But when they knew. When they perceived or ascertained.

That he was a Jew. There was a general prejudice against the Jews. They were disposed to charge the whole difficulty on Jews—esteeming Christians to be but a sect of the Jews. They were, therefore, indignant and excited, and indiscriminate in their wrath, and unwilling to listen to any defence.

With one voice. Unitedly; in one continued shout and clamour.

About the space of two hours. The day, from sunrise to sunset, among the Greeks and Romans, was divided into twelve equal parts (John 11:9). An hour, therefore, did not differ materially from an hour with us. It is not at all improbable that the tumult would continue for so long a time, before it would be possible to allay the excitement.

Cried out, etc. This they at first did to silence Alexander. The excitement, however, was continued in order to evince their attachment to Diana, as would be natural in an excited and tumultuous mob of debased heathen worshippers.