Don't cast pearls before swine. Taken from Matthew 7:8 " Give not that which is holy to the dogs, neither cast you your pearls before swine..."
Generally people translate this in terms of presenting sacred doctrine to those unready to hear it. That seems a little out of character and does not reflect the general mission ethos of the Gospels. That model would otherwise encourage sacrificing and witnessing to those who would not appreciate or value the message. So what is its meaning?
It is also in the context of the Sermon on the Mount, which defines the life of the God-person of the New Testament. The better question is not who are the "dogs" and the "pigs" but what are the "holy" and the "pearls"? Parables and wisdom statements are not intended to be taken literally but rather are representative of deeper and more nuanced truths.
2) Pigs will trample and destroy pearls. ibid. "lest they trample them under their feet, and turn again and rend you". This is really a cause and reaction statement. It has clearer meaning if read as 'don't cast those pearls before swine BECAUSE they will trample them under their feet...and then turn back to attack you.' You have been warned.
2) A dog will return to its vomit seems to fit as well - Proverbs 26.11 As a dog returns to his vomit, so a fool returns to his folly. Have you ever had a sore tooth or a wound you kept touching? You knew you should not but your tongue kept finding that tooth or you had to mess with that scab despite instructions to leave it alone. The dog goes back to smell, to lick, to sometimes re-consume that which it has divested itself. Short-term memory loss, a sense of ownership, or just really loose food tastes - "why" does not matter here only that it happens. Symbolically it was probably understood as a foolish action. We think today in terms of imprudent actions, mistakes, errors but an older definition uses terms such as evil ways, actions, sins.
3) What is "holy"? What are the "pearls"?