John Calvin Commentary Acts 22:18

"and saw him saying unto me, Make haste, and get thee quickly out of Jerusalem; because they will not receive of thee testimony concerning me." — Acts 22:18 (ASV)

Because they will not. Though the commandment of God alone ought to be sufficient to bind us to obey, yet so that Paul might be more willing to follow, Christ shows him a reason why He would have him depart from Jerusalem; namely, because he would lose his labor there.

But he was not chosen so that he might be idle, or accomplish nothing by teaching, even though this was a severe trial, and one that we may think greatly shook him.

Not long before, the task of preaching the gospel was entrusted to him, so that his voice might sound throughout the whole world. Now, even at the very outset, he is restrained. Indeed, his labor seems to be condemned with a unique reproach when his testimony is rejected because he himself is hated.

But it was fitting that the holy servant of the Lord should be humbled in this way, so that all preachers of the gospel might learn to surrender themselves entirely to obey Christ; so that when they are excluded from one place, they might be ready immediately to go to another; and so that they might not be discouraged, nor cease from doing their duty, even though they are undeservedly loathed.