When pious Jews left gentile cities and areas they might symbolically “shake the dust from their sandals and feet” as a sign of being separate from the people they had been mixing with.
Jesus takes this practice and commands the disciples to use this same sign when a household refuses to listen and accept their testimony that the “kingdom of God has come near”. It shows that the disciples are separate from the Jews and the people who rejected the testimony of Jesus had made a wrong choice.
Christians have a hard job. We are supposed to be “in” the world but not “of” the world. We are supposed to be separate. The Hebrew word for “Holy” is derived from the word for separate, different.
This is what led Peter to write “As obedient children, do not be conformed to the passions of your former ignorance, but as he who called you is holy, be holy yourselves in all your conduct” (1 Peter 1:14-15)
It is a hard thing to do of course and our example is Jesus himself. He never shunned anyone, or thought of people as too unclean or too far from the kingdom of God to mix with. He was famous after all for dining with prostitutes and tax collectors – a stick that was used to beat him with by his opponents.
But he was never corrupted by them. He was the salt and light that he commands us to be like. Instead of being made dirty by the company he kept, his light and goodness affected the people he mixed with.
In the world – in the sense that we are here to positively affect the people we meet and can influence, but certainly not “of” the world.
It is so easy to be a kind of chameleon Christian, adopting the language, morality, and attitudes of the company we keep, so we are Christian amongst Christians but not amongst our secular friends.
Another thing to note is that to be amongst antagonistic non believers for any length of time sapped the energy of even Jesus. His power was diminished amongst the people of his home town Nazareth. Not obliterated, but seriously impaired.
Shared belief and expectation and engagement amplified His power and message. That is so true today. Christians, who don’t spend much time with fellow Christians, find, after a while that the fire dims and can even go out completely. Mutual re-enforcement, encouragement and challenge are needed to keep fanning the flame of faith. Christianity is always corporate and not a private thing.
How true is that of our own life? Are we in the world but not of the world, or simply in the world. Has our salt lost its saltiness. Has our light gone out?