Matthew 5:21

"Ye have heard that it was said to them of old time, Thou shall not kill." And yet it was Himself who gave those laws also, but so far He states them impersonally. For if on the one hand He had said, "Ye have heard that I said to them of old," the saying would have been hard to receive, and would have stood in the way of all the hearers. If again, on the other hand, after having said, "Ye have heard that it was said to them of old by my Father," He had added, "But I say," He would have seemed to be taking yet more on Himself. Wherefore He hath simply stated it, making out thereby one point only; the proof that in fitting season He had come saying these things. For by the words, "It was said to them of old," He pointed out the length of the time, since they received this commandment. And this He did to shame the hearer, shrinking from the advance to the higher class of His commandments; as though a teacher should say to a child that was indolent, "Knowest thou not how long a time thou hast consumed in learning syllables?" This then He also covertly intimates by the expression, "them of old time," and thus for the future summons them on to the higher order of His instructions: as if He had said, "Ye are learning these lessons long enough, and you must henceforth press on to such as are higher than these." And it is well that He doth not disturb the order of the commandments, but begins first with that which comes earlier, with which the law also began. Yea, for this too suits with one showing the harmony between them.

(cont. Faust. xix. 30.) For almost all the precepts which the Lord gave, saying, But I say unto you, are found in those ancient books. But because they knew not of any murder, besides the destruction of the body, the Lord shows them that every evil thought to the hurt of a brother is to be held for a kind of murder. (De Civ. Dei, i. 20.) We do not, because we have heard that, Thou shall not kill, deem it therefore unlawful to pluck a twig, according to the error of the Manichees, nor consider it to extend to irrational brutes; by the most righteous ordinance of the Creator their life and death is subservient to our needs. There remains, therefore, only man of whom we can understand it, and that not any other man, nor you only; for he who kills himself does nothing else but kill a man. Yet have not they in any way done contrary to this commandment who have waged wars under God's authority, or they who charged with the administration of civil power have by most just and reasonable orders inflicted death upon criminals. Also Abraham was not charged with cruelty, but even received the praise of piety, for that he was willing to obey God in slaying his son. Those are to be excepted from this command whom God commands to be put to death, either by a general law given, or by particular admonition at any special time. For he is not the slayer who ministers to the command, like a hilt to one smiting with a sword, nor is Samson otherwise to be acquitted for destroying himself along with his enemies, than because he was so instructed privily of the Holy Spirit, who through him wrought the miracles.

Christ willing to show that He is the same God who spoke of old in the Law, and who now gives commandments in grace, now puts first of all his commandments (vid. Mat. 19:18.), that one which was the first in the Law, first, at least, of all those that forbade injury to our neighbour.