In Matthew 10 Jesus sends the Twelve out on their first evangelistic journey. Among the instructions he gives is this:
If anyone will not receive you or listen to your words, shake off the dust from your feet when you leave that house or town. Truly, I say to you, it will be more bearable on the day of judgment for the land of Sodom and Gomorrah than for that town.
What in the world are we supposed to do with this? I have no idea. Granted, not all Jesus’ instructions to the disciples apply to us in exactly the same way, and all that. But seriously– what do we do with this? Shouldn’t we keep telling people about the Gospel, knowing that God could change their heart any minute, even if they’re hostile to it? Don’t people get saved all the time after rejecting the Gospel over and over again?
I’m pretty sure I haven’t shared the Gospel long enough or often enough with anyone to be at this point. But in theory, what do you think? Is there ever a point when we “shake off the dust” and walk away?
Wiser Time 
I quickly ran to Barclay for some starting ideas as he is usually pretty straight to the point, if not a little embellishing at times… he says that the shaking of the dust was a specific instruction for that one mission the disciples were on becuase they had limited time to spread the “kingdom news.” (I’m not sure his reasoning on this as he doesn’t give it, but makes sense as Jesus commands elsewhere to “turn the other cheek” “walk the extra mile” “give your tunic” etc.) He also claims that it’s indicative of how some chances come and go, and we never get them back… (not sure how useful that is).
Also, I noticed Sodom and Gomorrah were mentioned, and they were destroyed for not being hospitable to God (but not the message of Christ). Interesting connection Jesus makes.