John Calvin Commentary Acts 9:21

"And all that heard him were amazed, and said, Is not this he that in Jerusalem made havoc of them that called on this name? and he had come hither for this intent, that he might bring them bound before the chief priests." — Acts 9:21 (ASV)

They were all amazed. This is added so that we may know that the power of God was acknowledged. Since Paul's zeal against the gospel was openly known, they saw no other cause for such a sudden change than the hand of God. Therefore, this is also one fruit of the miracle: that they all wondered at his being so suddenly made a new man, so that his teaching thereby moved their minds all the more. The report that he had raged horribly with great cruelty and had recently come to Damascus to carry out his purpose—these circumstances serve to magnify the miracle.

We must also note the phrase those who call on his name, which testifies that the godly professed the name of Christ in such a way that they placed all their hope of salvation in him. According to that, it is written:

Men put their trust in chariots, and others in horses, but we will call upon the name of the Lord,
(Psalms 20:7)

Finally, whatever the Scripture commands concerning calling upon the name of God applies to the person of Christ.