John Calvin Commentary Acts 19:20

"So mightily grew the word of the Lord and prevailed." — Acts 19:20 (ASV)

Grew mightily. The phrase κατα κρατος signifies that the word increased significantly (or that these developments were not common), as if the scriptural author would say that in these increases a rare efficacy appeared, one greater than was usual.

I refer the word grew to the number of believers, as if the scriptural author had said that the Church was increased by new disciples being gathered together daily, because doctrine was spread abroad. And I interpret that the word was confirmed in everyone as follows: namely, that they profited more and more in the obedience of the gospel and in godliness, and that their faith took deeper root.