Adam Clarke on Matthew 10:14

Shake off the dust of your feet - The Jews considered themselves defiled by the dust of a heathen country, when was represented by the prophets as a polluted laud, Amo 7:17, when compared with the land of Israel, which was considered as a holy land, Eze 45:1; therefore, to shake the dust of any city of Israel from off one's clothes or feet was an emblematical action, signifying a renunciation of all farther connection with them, and placing them on a level with the cities of the Heathen. See Amo 9:7.