Give not that which is holy to dogs; neither cast ye your pearls before swine
"As of all the sacred mysteries … none can compare with the … Eucharist, so likewise for no crime is there heavier punishment to be feared from God than for the unholy or irreligious use by the faithful of that which … contains the very Author and Source of holiness." - Catechism of the Council of Trent (De Euch., v.i)
PSEUDO-CHRYSOSTOM. (Catena Aurea) Otherwise; The Lord had commanded us to love our enemies, and to do good to those that sin against us. That from this Priests might not think themselves obliged to communicate also the things of God to such, He checked any such thought saying, Give not that which is holy to the dogs; as much as to say, I have bid you love your enemies, and do them good out of your temporal goods, but not out of My spiritual goods, without distinction. For they are your brethren by nature but not by faith, and God gives the good things of this life equally to the worthy and the unworthy, but not so spiritual graces.
AUGUSTINE. (ubi sup.) The dogs are those that assault the truth; the swine we may not unsuitably take for those that despise the truth. Therefore because dogs leap forth to rend in pieces, and what they rend, suffer not to continue whole, He said, Give not that which is holy to the dogs; because they strive to the utmost of their power to destroy the truth. The swine though they do not assault by biting as dogs, yet do they defile by trampling upon, and therefore He said, Cast not your pearls before swine.
RABANUS. Or; The dogs are returned to their vomit; the swine not yet returned, but wallowing in the mire of vices.
PSEUDO-CHRYSOSTOM. Otherwise; The dog and the swine are unclean animals; the dog indeed in every respect, as he neither chews the cud, nor divides the hoof; but swine in one respect only, seeing they divide the hoof, though they do not chew the cud. Hence I think that we are to understand by the dog, the Gentiles who are altogether unclean, both in their life, and in their faith; but by the swine are to be understood heretics, because they seem to call upon the name of the Lord. Give not therefore that which is holy to the dogs, for that baptism and the other sacraments are not to be given but to them that have the faith. In like manner the mysteries of the truth, that is, the pearls, are not to be given but to such as desire the truth and live with human reason. If then you cast them to the swine, that is, to such as are grovelling in impurity of life, they do not understand their preciousness, but value them like to other worldly fables, and tread them under foot with their carnal life.
New Sacraments: "The False Prophet may institute secret rites, through which the followers of Antichrist will be advanced by degrees into the deeper mysticism of his cult. A sort of diabolical sacramental system would thus be instituted conferring the graces of Satan and consecrating people to the service of the Beast." page 325