Luke 9:5

Ver. 5. The gospel does not force itself upon men; it is an elastic power, penetrating wherever it finds access, and retiring wherever it is repulsed. This was Jesus' own mode of acting all through His ministry (Luke 8:37; John 3:22).

The Jews were accustomed, on their return from heathen countries to the Holy Land, to shake off the dust from their feet at the frontier. This act symbolized a breaking away from all joint-participation in the life of the idolatrous world. The apostles were to act in the same way in reference to any Jewish cities which might reject in their person the kingdom of God. Καί, even the dust. By this symbolical act they relieved themselves of the burden of all further responsibility on account of the people of that city.

The expression, for a testimony, with the complement ἐπ᾿ αὐτούς, upon them, has evidently reference to the judgment to come; in Mark, the complement αὐτοῖς, for them, makes the testimony an immediate appeal to their guilty consciences.