A.T. Robertson Commentary Mark 6:11

"And whatsoever place shall not receive you, and they hear you not, as ye go forth thence, shake off the dust that is under your feet for a testimony unto them." — Mark 6:11 (ASV)

For a testimony unto them (εις μαρτυριον αυτοις). Not in Matthew. Luke 9:5 has "for a testimony against them" (εις μαρτυριον επ αυτους). The dative αυτοις in Mark is the dative of disadvantage and really carries the same idea as επ in Luke. The dramatic figure of

shaking out (εκτιναξατε, effective aorist imperative, Mark and Matthew),

shaking off (αποτινασσετε, present imperative, Luke).