Matthew 10:14 And whosoever shall not receive you, nor hear your words, when ye depart out of that house or city, shake off the dust of your feet.
To obtain a complete understand of what the Lord is telling us about witnessing Christ to the lost, we need to compare the shaking off of dust from our feet with His example of washing the feet of the disciples.
One of the aspects of washing of the feet is forgiveness of sins. This is a cleansing process. Forgiveness is a cleansing process. Here in Matthew He instructs us to shake off the dust. This is not a cleansing, it is not a matter of forgiveness. A shaking off is also indicative that it is superficial, not caked on, not filth.
But it is still dust, dirt, and the results of touching uncleanness. So it still represents sin, but not our sin. When you witness to the lost and they do not receive the word of salvation those words fall to the ground and the dust that is kicked up, the sin, is a result of their rejected the Word, rejecting Jesus. This is not your sin, it is theirs.
The reason Jesus said to shake it off is to tell us not to take it personally. Do not take it upon yourself. We spend time and energy witnessing and when they reject Jesus we may mourn for them. To mourn is fine, but to feel rejected is not.
Luke 6:22 Blessed are ye, when men shall hate you, and when they shall separate you from their company, and shall reproach you, and cast out your name as evil, for the Son of man’s sake.
For His sake, don’t take it personally. Shake it off.