21 “For I say unto you, that unless your righteousness shall exceed the righteousness of the scribes and Pharisees, you shall in no case enter into the kingdom of heaven;” i.e., unless you shall fulfil not only those least precepts of the law which begin the man, but also those which are added by me, who am not come to destroy the law, but to fulfil it, you shall not enter into the kingdom of heaven. But you say to me: If, when He was speaking above of those least commandments, He said that whosoever shall break one of them, and shall teach in accordance with his transgression, is called the least in the kingdom of heaven; but that whosoever shall do them, and shall teach [men] so, is called great, and hence will be already in the kingdom of heaven, because he is great: what need is there for additions to the least precepts of the law, if he can be already in the kingdom of heaven, because whosoever shall do them, and shall so teach, is great? For this reason that sentence is to be understood thus: “But whosoever shall do and teach men so, the same shall be called great in the kingdom of heaven,”— i.e. not in accordance with those least commandments, but in accordance with those which I am about to mention. Now what are they? “That your righteousness,” says He, “may exceed that of the scribes and Pharisees;” for unless it shall exceed theirs, you shall not enter into the kingdom of heaven. Whosoever, therefore, shall break those least commandments, and shall teach men so, shall be called the least; but whosoever shall do those least commandments, and shall teach men so, is not necessarily to be reckoned great and meet for the kingdom of heaven; but yet he is not so much the least as the man who breaks them. But in order that he may be great and fit for that kingdom, he ought to do and teach as Christ now teaches, i.e. in order that his righteousness may exceed that of the scribes and Pharisees. The righteousness of the Pharisees is, that they shall not kill; the righteousness of those who are destined to enter into the kingdom of God, that they be not angry without a cause. The least commandment, therefore, is not to kill; and whosoever shall break that, shall be called least in the kingdom of heaven; but whosoever shall fulfil that commandment not to kill, will not, as a necessary consequence, be great and meet for the kingdom of heaven, but yet he ascends a certain step. He will be perfected, however, if he be not angry without a cause; and if he shall do this, he will be much further removed from murder. For this reason he who teaches that we should not be angry, does not break the law not to kill, but rather fulfils it; so that we preserve our innocence both outwardly when we do not kill, and in heart when we are not angry.
22. “You have heard” therefore, says He, “that it was said to them of old time, You shall not kill; and whosoever shall kill shall be in danger of the judgment. But I say unto you, that whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, You fool, shall be in danger of the gehenna of fire.” What is the difference between being in danger of the judgment, and being in danger of the council, and being in danger of the gehenna of fire? For this last sounds most weighty, and reminds us that certain stages were passed over from lighter to more weighty, until the gehenna of fire was reached. And, therefore, if it is a lighter thing to be in danger of the judgment than to be in danger of the council, and if it is also a lighter thing to be in danger of the council than to be in danger of the gehenna of fire, we must understand it to be a lighter thing to be angry with a brother without a cause than to say “Raca;” and again, to be a lighter thing to say “Raca” than to say “Thou fool.” For the danger would not have gradations, unless the sins also were mentioned in gradation.
23. But here one obscure word has found a place, for “Raca” is neither Latin nor Greek. The others, however, are current in our language. Now, some have wished to derive the interpretation of this expression from the Greek, supposing that a ragged person is called “Raca,” because a rag is called in Greek ῥάκος; yet, when one asks them what a ragged person is called in Greek, they do not answer “Raca;” and further, the Latin translator might have put the word ragged where he has placed “Raca,” and not have used a word which, on the one hand, has no existence in the Latin language, and, on the other, is rare in the Greek. Hence the view is more probable which I heard from a certain Hebrew whom I had asked about it; for he said that the word does not mean anything, but merely expresses the emotion of an angry mind. Grammarians call those particles of speech which express an affection of an agitated mind interjections; as when it is said by one who is grieved, “Alas,” or by one who is angry, “Hah.” And these words in all languages are proper names, and are not easily translated into another language; and this cause certainly compelled alike the Greek and the Latin translators to put the word itself, inasmuch as they could find no way of translating it.
Source: Our Lord's Sermon on the Mount (New Advent)