For a testimony unto them (ei marturion autoi). Not in Matthew. Luke 9:5 has "for a testimony against them" (ei marturion epi autou). The dative autoi in Mark is the dative of disadvantage and really carries the same idea as epi in Luke. The dramatic figure of shaking out (ektinaxate, effective aorist imperative, Mark and Matthew), shaking off (apotinassete, present imperative, Luke).