John Calvin on Acts 13:51

51. When they had shaken of the dust of their feet. We may also gather, even by the commandment of Christ, (Matthew 10:14; Luke 9:5; 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, forasmuch as it was his purpose to terrify the gross and professed condemners of his doctrine. Furthermore, he meant by this means to declare that God doth so detest the wicked, that we must take meat 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 whereon they tread; but the Lord did never command that any, save only the condemners of his word, should be so rejected with such execration. If any adulterer or whoremonger, if any perjured person, if any drunkard, “Si quis homicida,” if any murderer, omitted. were to be excommunicate, this sign was not used. Therefore, it appeareth how intolerable the contempt of the word of God is in his sight; because, when as he commandeth that the dust of the feet be shaken off, it is as much as if he should pronounce that they are the bond-slaves of Satan, men past hope, and worthy to be banished “Qui exterminentur,” to be exterminated.

from off the earth. Wherefore, let this so great;, severity teach us to reverence the gospel. Also the ministers of the word are taught with how great ferventness of zeal they must maintain the majesty of the word, that they do not coldly dissemble and wink at the contempt thereof.