http://www.ccel.org/ccel/mcgarvey/gospels.vii.xii.html
a 6 Give not that which is holy unto the dogs, neither cast your pearls before the swine, lest haply they trample them under their feet, and turn and rend you.
The commentary makes a point that I had not thought of here – that the facilities and persons of the church (the holy) should not be used for that which is unholy (the dogs). For example, we should not open a saloon with a prayer of blessing. And a pastor should not marry people if it would cause an adulterous marriage. And here’s one: casting your pearls before swine means you need to be able to recognize when someone is not receptive to your message and is in fact belligerent. Jesus says to leave them alone, in the same way that the early Christians left the Jews alone so they could preach to the Gentiles. I don’t know what that means exactly; i.e., if you present the gospel message to someone three times and they aren’t interested, then you give up? I don’t think that is it – I think it’s more like someone who actively fights against you. I think Jesus is saying that it’s better to move on than to stay and argue.