What does Jesus mean by not casting pearls before swine?
Jesus means there is no use wasting your breath on someone who is absolutely contrary, rebellious, and indifferent to the things of the Word. At the same time, we should present God's plan of salvation to everyone, trusting that God knows who will respond and opens the heart.
Les explains this in the context of foreknowledge. Jesus' statement about not casting pearls before swine means there is no use wasting your breath on someone who is absolutely contrary, rebellious, and indifferent to the things of the Word. Yet we still have to approach and do our best to present God's plan of salvation to anyone and everyone. God knows which ones are going to respond, and knowing that, He opens the heart. This is the beginning step of coming into salvation: a body and soul with nothing going for us but a sin nature, the spirit dead, and dead people cannot even believe anything. As the Word continues to go out, God the Holy Spirit lights a spark within an individual, and it may take time before that individual finally comes to really believing.
Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you.