Luke 9:5

“And as many as do not receive you, when you depart from that city, shake off the dust from your feet for a testimony against them.”

But where none in a town would receive them, when they left that town they should shake the dust of the town from their feet (just as pious Jews did when they left Gentile territory, although for a different ritual reason). That action would stand as a testimony to that town's rejection of God and would declare that they were cut off from the new Israel and were outsiders like the Gentiles. It would count against them in the Judgment. So their going forth was with both mercy and judgment. For those who received them, mercy, For those who would not receive them, judgment.