Mark 6:11
And if any place will not welcome you or listen to you, leave that place and shake the dust off your feet as a testimony against them — the dust-shaking is a symbolic act of testimony: leaving behind even the dust of an unwelcoming place communicates complete disassociation. Jewish travelers returning from Gentile territory would shake the dust from their sandals to avoid bringing impurity into the land; Jesus' instruction reverses this — the disciples shake off the dust of a Jewish town that has refused the kingdom's message. The act is a testimony against them — a public declaration that they have had the opportunity and refused it.