Matthew 7:6
"Do not give dogs what is holy, and do not throw your pearls before pigs, lest they trample them underfoot and turn to attack you.
General
The main idea of this text is we must be judicious with our words. Note Paul's prayer to be delivered from the wicked:
2 Thessalonians 3:1-2
Finally, brothers, pray for us, that the word of the Lord may speed ahead and be honored, as happened among you, and that we may be delivered from wicked and evil men. For not all have faith.
Note, too, the similarity between Jesus's subjects (dogs and pigs) and Peter's warning near the end of his writings. Peter sees the illustration as a warning against trusting in false teachers, whose teaching will lead us into a life of sin all over again. What the Proverbs call "folly" is evil.
2 Peter 2:22
What the true proverb says has happened to them: “The dog returns to its own vomit, and the sow, after washing herself, returns to wallow in the mire.”
Proverbs 26:11
Like a dog that returns to his vomit
is a fool who repeats his folly.
The prodigal son in Luke certainly chose a life of sin over a life that would honor his father. As low in the order of things as pigs may be, the prodigal son was lower:
Luke 15:16
And he was longing to be fed with the pods that the pigs ate, and no one gave him anything.
do not give dogs what is holy
The Law specifically draws a distinction between what holy Israelites may eat and what should be left for the dogs instead:
Exodus 22:31
You shall be consecrated to me. Therefore you shall not eat any flesh that is torn by beasts in the field; you shall throw it to the dogs.
But the Law also introduces the concept of "dogs" as a metaphor for male cult prostitutes.
Deuteronomy 23:18
You shall not bring the fee of a prostitute or the wages of a dog into the house of the Lord your God in payment for any vow, for both of these are an abomination to the Lord your God.
Paul extends the use of "dog" more generally to any who think to serve God but actually do evil.
Philippians 3:2
Look out for the dogs, look out for the evildoers, look out for those who mutilate the flesh.
All these references distinguish what is appropriate from what is not appropriate.
do not throw your pearls before pigs
A herd of pigs is quite a serious threat to a person's safety; many farmworkers have disappeared while tending to them.
The illustration here is of feeding time. The pigs are expecting their normal fare, and pearls may at first look like pea pods or soy beans to them, but they will quickly realize pearls are not food. In their hunger-driven madness, they may stampede and kill a person to feed themselves.
Why would someone cast pearls before them in the first place? Perhaps they fear being trampled and offer up pearls as a ransom. Perhaps they want to treat the pigs, thinking pearls more desirable than soy beans.
In short, it's a waste of time to try placating a herd of swine with valuables, and it just might anger them instead!