John Calvin Commentary Acts 2:41

"They then that received his word were baptized: and there were added [unto them] in that day about three thousand souls." — Acts 2:41 (ASV)

They, therefore, which willingly. Luke shows more plainly how fruitful this one sermon Peter delivered was: namely, that it gained for Christ about three thousand men. And with it, he declares the nature and force of faith when he says that with a prompt and ready mind they embraced his word.

Therefore, faith must begin with this readiness and willing desire to obey. And because many show themselves very willing at first, who afterward lack constancy or perseverance, lest we should think that it was some sudden impulse which soon fell away, Luke also afterward commends their constancy. He shows that those who (as he said) willingly embraced this word of the apostles were joined to the disciples, or were grafted into the same body, and that they continued in their doctrine.

Therefore, we must neither be slow to obey, nor swift to draw back; but we must hold fast and stand firmly to that doctrine which we immediately (without any delay) embraced.

Furthermore, this example ought to make us quite ashamed. For whereas a great multitude was converted to Christ with one sermon, a hundred sermons can scarcely move a few of us. And whereas Luke says that they continued, there is scarcely one among ten who shows even a moderate desire to profit and go forward; indeed, the greater part soon loathes our doctrine. Woe, therefore, to the sluggishness and fickleness of the world!