Luke 9:5

For it is very improbable that those who despise the saving Word, and the Master of the household, will show themselves kind to His servants, and seek further blessings.

The dust is shaken off from the Apostles' feet as a testimony of their labours, that they entered into a city, and the apostolical preaching had reached to the inhabitants thereof. Or the dust is shaken off when they receive nothing (not even of the necessaries of life) from those who despised the Gospel. But if any by treacherous negligence, or even from zeal, despise the word of God, their communion must be shunned, the dust of the feet must be shaken off, lest by their vain deeds which are to be compared to the dust, the footstep of a chaste mind be defiled.

He pronounces it to be foreign to the character of a preacher of the heavenly kingdom to run from house to house and change the rights of inviolable hospitality; but as the grace of hospitality is supposed to be offered, so also if they are not received the dust must be shaken off, and they are commanded to depart from the city; as it follows, And whosoever will not receive you when ye go out of that city, shake off the very dust from your feet for a testimony, &c. Or it is a great return of hospitality which is here taught, i. e. that we should not only wish peace to our hosts, but also if any faults of earthly infirmity obscure them, they should be removed by receiving the footsteps of apostolical preaching.