“But into whatever city you shall enter, and they receive you not, go out into its streets and say, ‘Even the dust from your city, which cleaves to our feet, we wipe off against you, nevertheless know this, that the Kingly Rule of God is come near.”
After they have made every attempt to evangelise a city or town, if they find themselves ‘not received' they are to go out into the streets of the city and publicly shake the dust from their feet. (Their non-reception might be revealed among other ways by their being beaten in the synagogue, a treatment meted out to ‘heretics'). This shaking of the dust from the feet was something all pious Jews did on leaving Gentile territory, because such territory was seen as ‘ritually unclean'. Here it would be an indication that that city or town was to be seen as unclean by God. They were thereby cut off from the covenant. They no longer belonged to Israel. They were effectively under a curse. So while Jesus did not approve of immediately bringing down fire on men, He did recognise that they could be committed to the future judgment of God.
At the same time they were to announce to them the reason why they did it. It was because the Kingly Rule of God had approached them, but they had rejected it, and thus God had rejected them. It would thus still give opportunity to any seeking heart to seek them out and receive their message, and enter under the Kingly Rule of God.