Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 6:6-13

“That is, lest they should be accused of gluttony in passing from one to another. It goes on, And whoever shall not receive you, &c. This the Lord commanded them, that they might shew that they had walked a long way for their sakes, and to no purpose. Or, because they received nothing from them, not even dust, which they shake off, that it might be a testimony against them, that is, by way of convicting themv.”