John Calvin 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)

Great is Diana of the Ephesians. This was a clamorous confession, but without any soundness; nor did it proceed from the faith of the heart. For from where did that great divinity of Diana, of which they spoke, come, except that, like madmen, they furiously defended the error they had once received?

It is different with true godliness, in that we believe with the heart for righteousness, and then the confession of the mouth follows for salvation. In this, the derangement and mad stubbornness of all madmen and deluded fools differ from the constancy and zeal of the martyrs.

And yet, nevertheless, our sluggishness is shameful if we are not as ready and resolute in the confession of a sure faith as they are in their vile error. For we see what the Spirit of God prescribes for us by the mouth of David:

“I believed, and therefore will I speak,” (Psalms 116:10).