Calvin's Commentaries, Vol. 31: Matthew, Mark and Luke, Part I, tr. by John King, [1847-50], at sacred-texts.com
6. Give not that which is holy to the dogs, and do not throw down your pearls before swine, lest these trample them under their feet, and those turn and tear you.
6. Give not that which is holy It is unnecessary to repeat oftener, that Matthew gives us here detached sentences, which ought not to be viewed as a continued discourse. The present instruction is not at all connected with what came immediately before, but is entirely separate from it. Christ reminds the Apostles, and, through them, all the teachers of the Gospel, to reserve the treasure of heavenly wisdom for the children of God alone, and not to expose it to unworthy and profane despisers of his word.
But here a question arises: for he afterwards commanded to preach the Gospel to every creature, (Mr 16:15;) and Paul says, that the preaching of it is a deadly savor to wicked men, (2Co 2:16;) and nothing is more certain than that it is every day held out to unbelievers, by the command of God, for a testimony, that they may be rendered the more inexcusable. I reply: As the ministers of the Gospel, and those who are called to the office of teaching, cannot distinguish between the children of God and swine, it is their duty to present the doctrine of salvation indiscriminately to all. Though many may appear to them, at first, to be hardened and unyielding, yet charity forbids that such persons should be immediately pronounced to be desperate. It ought to be understood, that dogs and swine are names given not to every kind of debauched men, or to those who are destitute of the fear of God and of true godliness, but to those who, by clear evidences, have manifested a hardened contempt of God, so that their disease appears to be incurable. In another passage, Christ places the dogs in contrast with the elect people of God and the household of faith, It is not proper to take the children’s bread, and give it to dogs, (Mt 15:27.) But by dogs and swine he means here those who are so thoroughly imbued with a wicked contempt of God, that they refuse to accept any remedy.
Hence it is evident, how grievously the words of Christ are tortured by those who think that he limits the doctrine of the Gospel to those only who are teachable and well-prepared. For what will be the consequence, if nobody is invited by pious teachers, until by his obedience he has anticipated the grace of God? On the contrary, we are all by nature unholy, and prone to rebellion. The remedy of salvation must be refused to none, till they have rejected it so basely when offered to them, as to make it evident that they are reprobate and self-condemned, (αὐτοκατάκριτοι,) as Paul says of heretics, (Titus 3:11.)
There are two reasons, why Christ forbade that the Gospel should be offered to lost despisers. It is an open profanation of the mysteries of God to expose them to the taunts of wicked men. Another reason is, that Christ intended to comfort his disciples, that they might not cease to bestow their labors on the elect of God in teaching the Gospel, though they saw it wantonly rejected by wicked and ungodly men. His meaning is lest this inestimable treasure should be held in little estimation, swine and dogs must not be permitted to approach it. There are two designations which Christ bestows on the doctrine of salvation: he calls it holy, and compares it to pearls. Hence we learn how highly we ought to esteem this doctrine.
Lest these trample them under their feet Christ appears to distinguish between the swine and the dogs: attributing brutal stupidity to the swine, and rage to the dogs And certainly, experience shows, that there are two such classes of despisers of God. Whatever is taught in Scripture, for instance, about the corrupt nature of man, free justification, and eternal election, is turned by many into an encouragement to sloth and to carnal indulgence. Such persons are fitly and justly pronounced to be swine Others, again, tear the pure doctrine, and its ministers, with sacrilegious reproaches, as if they threw away all desire to do well, all fear of God, and all care for their salvation. Although he employs both names to describe the incurable opponents of the Word of God, yet, by a twofold comparison, he points out briefly in what respect the one differs from the other.