The schools of the scribes taught that the dust of heathen lands was defiling. They therefore objected even to bringing plants or herbs from heathen countries, lest some of the dust should come with them. Some of the rabbins permitted this, provided no dust was brought with the plants. They give this gloss to the rule: “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.” See LIGHTFOOT, flora Hebraicae.
The Saviour, doubtless, alluded to this rabbinical rule, and, by using the expression of the text, conveyed the idea to his disciples that every place which should reject them was to be considered heathen, impure, profane.
When Paul and Barnabas were driven from Antioch, in Pisidia, “they shook of the dust of their feet.” See Acts 13:5151But they shook off the dust of their feet against them, and came unto Iconium. (Acts 13:51).