John Calvin Commentary Acts 13:51

"But they shook off the dust of their feet against them, and came unto Iconium." — Acts 13:51 (ASV)

When they had shaken off the dust of their feet. We may also gather, even by the commandment of Christ (Matthew 10:14; Luke 9:5; Luke 10:11), that this was a token of cursing among the Jews. For it is not to be thought that Christ meant to have His disciples use an unknown sign, since it was His purpose to terrify the gross and professed contemners of His doctrine.

Furthermore, He meant by this means to declare that God so detests the wicked, that we must take great heed that we have no fellowship with them, lest we be infected with their uncleanness. All the wicked are said, indeed, to pollute the ground on which they tread; but the Lord never commanded that any, save only the contemners of His word, should be so rejected with such execration.

If any adulterer or whoremonger, if any perjured person, if any drunkard, were to be excommunicated, this sign was not used. Therefore, it appears how intolerable the contempt of the Word of God is in His sight; because, when He commands that the dust of the feet be shaken off, it is as much as if He pronounced that they are the bond-slaves of Satan, men past hope, and worthy to be banished from the earth.

Therefore, let this so great severity teach us to reverence the gospel. Also, the ministers of the Word are taught with what great fervor of zeal they must maintain the majesty of the Word, so that they do not coldly dissemble and wink at contempt for it.