John Lightfoot on Matthew 10:14

[Shake off the dust of your feet.] The schools of the scribes taught that the dust of the heathen land defiled by the touch. "The dust of Syria defiles, as well as the dust of other heathen countries."

"A tradition-writer saith, 'They bring not herbs into the land of Israel out of a heathen land: but our Rabbins have permitted it.' What difference is there between these? R. Jeremiah saith, The care of their dust is among them." The Gloss is, "They take care, lest, together with the herbs, something of the dust of the heathen land be brought, which defiles in the tent, and defiles the purity of the land of Israel."

"By reason of six doubts, they burn the truma: the doubt of a field, in which heretofore might be a sepulchre; the doubt of dust brought from a heathen land," &c. Where the Gloss is this; "Because it may be doubted of all the dust of a heathen land, whether it were not from the sepulchre of the dead."

"Rabbi saw a certain priest standing in a part of the city Aco, which part was without the bounds of the land of Israel: he said to him, 'Is not that heathen land concerning which they have determined that it is as unclean as a burying-place?'"

Therefore that rite of shaking the dust off the feet, commanded the disciples, speaks thus much; "Wheresoever a city of Israel shall not receive you, when ye depart, shew, by shaking off the dust from your feet, that ye esteem that city, however a city of Israel, for a heathen, profane, impure city; and, as such, abhor it."