In the racial divide between Jews and Gentiles in the ancient world, the Jews would shake the dust off their feet after traveling through a Gentile town.  Jesus sent the 12 out to Jewish villages to preach.  The acceptance or rejection of the Kingdom message ultimately revealed the true people of God, so Jesus suggested that the gesture of “shaking the dust off” be used against Jews.  The rejection of the Kingdom is tragic.  The acceptance of the Kingdom is saving.  There is no partiality with God.  

“If any place will not welcome you and they refuse to hear you, as you leave, shake off the dust that is on your feet as a testimony against them.” Mark 6:11 (NRSV)