70 Then, further, when our Lord is found to have made certain statements which many who were present did not accept, but either resisted or despised, He is not to be thought to have given that which is holy to the dogs, or to have cast pearls before swine: for He did not give such things to those who were not able to receive them, but to those who were able, and were at the same time present; whom it was not meet that He should neglect on account of the impurity of others.
And when tempters put questions to Him, and He answered them, so that they might have nothing to gainsay, although they might pine away from the effects of their own poisons, rather than be filled with His food, yet others, who were able to receive His teaching, heard to their profit many things in consequence of the opportunity created by these parties. I have said this, lest any one, perhaps, when he is not able to reply to one who puts a question to him, should seem to himself excused, if he should say that he is unwilling to give that which is holy to the dogs, or to cast pearls before swine.
For he who knows what to answer ought to do it, even for the sake of others, in whose minds despair arises, if they believe that the question proposed cannot be answered: and this in reference to matters that are useful, and that belong to saving instruction. For many things which may be the subject of inquiry on the part of idle people are needless and vain, and often hurtful, respecting which, however, something must be said; but this very point is to be opened up and explained, viz. why such things ought not to form the subject of inquiry.
In reference, therefore, to things that are useful, we ought sometimes to give a reply to what is asked of us: just as the Lord did, when the Sadducees had asked Him about the woman who had seven husbands, to which of them she would belong in the resurrection. For He answered that in the resurrection they will neither marry, nor be given in marriage, but will be as the angels in heaven. But sometimes, he who asks is to be asked something else, by telling which he would answer himself as to the matter he asked about; but if he should refuse to make a statement, it would not seem to those who are present unfair, if he himself should not hear anything as to the matter he inquired about.
For those who put the question, tempting Him, whether tribute was to be paid, were asked another question, viz. whose image the money bore which was brought forward by themselves; and because they told what they had been asked, i.e. that the money bore the image of Cæsar, they gave a kind of answer to themselves in reference to the question they had asked the Lord: and accordingly from their answer He drew this inference, “Render therefore unto Cæsar the things which are Cæsar's, and unto God the things that are God's.” When, however, the chief priests and elders of the people had asked by what authority He was doing those things, He asked them about the baptism of John: and when they would not make a statement which they saw to be against themselves, and yet would not venture to say anything bad about John, on account of the bystanders, “Neither tell I you,” says He, “by what authority I do these things;” a refusal which appeared most just to the bystanders.
For they said they were ignorant of that which they really knew, but did not wish to tell. And, in truth, it was right that they who wished to have an answer to what they asked, should themselves first do what they required to be done toward them; and if they had done this, they would certainly have answered themselves. For they themselves had sent to John, asking who he was; or rather they themselves, being priests and Levites, had been sent, supposing that he was the very Christ, but he said that he was not, and gave forth a testimony concerning the Lord: a testimony respecting which if they chose to make a confession, they would teach themselves by what authority as the Christ He was doing those things; which as if ignorant of they had asked, in order that they might find an avenue for calumny.
Source: Our Lord's Sermon on the Mount (New Advent)