John Calvin Commentary Acts 16:18

"And this she did for many days. But Paul, being sore troubled, turned and said to the spirit, I charge thee in the name of Jesus Christ to come out of her. And it came out that very hour." — Acts 16:18 (ASV)

Paul took it grievously. It's possible that at first Paul ignored, and did not pay much attention to, the maid's shouting, because he hoped that no notice would be taken of it and preferred that it would disappear on its own. But the constant repetition eventually made him weary, because if he had feigned indifference for too long, Satan would have grown more and more insolent through his silence and patience.

Secondly, he should not have issued this prohibition rashly until he knew for certain that he was equipped with the power of God. For Paul’s command would have been foolish and futile without God's command. And this must be noted, so that no one condemns Paul for excessive haste because he confronted the unclean spirit so valiantly.

For he did not feel any grief or indignation, except that he saw Satan's influence would grow unless he quickly prevented it. Nor did he attempt anything without the prompting of the Spirit, nor did he enter the conflict until he was armed with power from heaven.

Nevertheless, he seems to contradict himself, since he says elsewhere that he rejoices whenever he sees the gospel preached (Philippians 1:18), even by wicked men and those who deliberately tried to bring him into contempt. I answer that he had another, more fitting reason in this situation: everyone would have thought that the maid's spirit was toying with Paul. Consequently, the doctrine of the gospel would not only have fallen under suspicion but also have come into great contempt. It was for this reason that Christ also commanded the devil to be silent (Mark 1:25), even though He nevertheless allowed His name to be praised by unsuitable and unworthy men (Luke 4:35).

I command thee. We must note the manner of speech, for the miracle was intended to have a twofold purpose: first, that the power of Christ might be known, and secondly, that Paul might declare he had no fellowship with Satan’s deceptions. So Paul, by attributing the authority and power to Christ alone, declares that he is only a minister.

Having done that, he openly sets Christ against the devil, so that, through the conflict, everyone may see that they are deadly enemies. For it was beneficial that many should be awakened who had been given over to such gross deception, so that, being thoroughly cleansed, they might come to the true faith.