Objection 1. It would seem that the precepts of the decalogue are dispensable. For the precepts of the decalogue belong to the natural law. But the natural law fails in some cases and is changeable, like human nature, as the Philosopher says (Ethic. v, 7). Now the failure of law to apply in certain particular cases is a reason for dispensation, as stated above (96, 6; 97, 4). Therefore a dispensation can be granted in the precepts of the decalogue.
Objection 2. Further, man stands in the same relation to human law as God does to Divine law. But man can dispense with the precepts of a law made by man. Therefore, since the precepts of the decalogue are ordained by God, it seems that God can dispense with them. Now our superiors are God's viceregents on earth; for the Apostle says (2 Corinthians 2:10): "For what I have pardoned, if I have pardoned anything, for your sakes have I done it in the person of Christ." Therefore superiors can dispense with the precepts of the decalogue.
Objection 3. Further, among the precepts of the decalogue is one forbidding murder. But it seems that a dispensation is given by men in this precept: for instance, when according to the prescription of human law, such as evil-doers or enemies are lawfully slain. Therefore the precepts of the decalogue are dispensable.
Objection 4. Further, the observance of the Sabbath is ordained by a precept of the decalogue. But a dispensation was granted in this precept; for it is written (1 Maccabees 2:4): "And they determined in that day, saying: Whosoever shall come up to fight against us on the Sabbath-day, we will fight against him." Therefore the precepts of the decalogue are dispensable.
On the contrary, are the words of Isaiah 24:5, where some are reproved for that "they have changed the ordinance, they have broken the everlasting covenant"; which, seemingly, apply principally to the precepts of the decalogue. Therefore the precepts of the decalogue cannot be changed by dispensation.
I answer that, As stated above (96, 6; 97, 4), precepts admit of dispensation, when there occurs a particular case in which, if the letter of the law be observed, the intention of the lawgiver is frustrated. Now the intention of every lawgiver is directed first and chiefly to the common good; secondly, to the order of justice and virtue, whereby the common good is preserved and attained. If therefore there by any precepts which contain the very preservation of the common good, or the very order of justice and virtue, such precepts contain the intention of the lawgiver, and therefore are indispensable. For instance, if in some community a law were enacted, such as this--that no man should work for the destruction of the commonwealth, or betray the state to its enemies, or that no man should do anything unjust or evil, such precepts would not admit of dispensation. But if other precepts were enacted, subordinate to the above, and determining certain special modes of procedure, these latter precepts would admit of dispensation, in so far as the omission of these precepts in certain cases would not be prejudicial to the former precepts which contain the intention of the lawgiver. For instance if, for the safeguarding of the commonwealth, it were enacted in some city that from each ward some men should keep watch as sentries in case of siege, some might be dispensed from this on account of some greater utility.
Now the precepts of the decalogue contain the very intention of the lawgiver, who is God. For the precepts of the first table, which direct us to God, contain the very order to the common and final good, which is God; while the precepts of the second table contain the order of justice to be observed among men, that nothing undue be done to anyone, and that each one be given his due; for it is in this sense that we are to take the precepts of the decalogue. Consequently the precepts of the decalogue admit of no dispensation whatever.
Reply to Objection 1. The Philosopher is not speaking of the natural law which contains the very order of justice: for it is a never-failing principle that "justice should be preserved." But he is speaking in reference to certain fixed modes of observing justice, which fail to apply in certain cases.
Reply to Objection 2. As the Apostle says (2 Timothy 2:13), "God continueth faithful, He cannot deny Himself." But He would deny Himself if He were to do away with the very order of His own justice, since He is justice itself. Wherefore God cannot dispense a man so that it be lawful for him not to direct himself to God, or not to be subject to His justice, even in those matters in which men are directed to one another.
Reply to Objection 3. The slaying of a man is forbidden in the decalogue, in so far as it bears the character of something undue: for in this sense the precept contains the very essence of justice. Human law cannot make it lawful for a man to be slain unduly. But it is not undue for evil-doers or foes of the common weal to be slain: hence this is not contrary to the precept of the decalogue; and such a killing is no murder as forbidden by that precept, as Augustine observes (De Lib. Arb. i, 4). In like manner when a man's property is taken from him, if it be due that he should lose it, this is not theft or robbery as forbidden by the decalogue.
Consequently when the children of Israel, by God's command, took away the spoils of the Egyptians, this was not theft; since it was due to them by the sentence of God. Likewise when Abraham consented to slay his son, he did not consent to murder, because his son was due to be slain by the command of God, Who is Lord of life and death: for He it is Who inflicts the punishment of death on all men, both godly and ungodly, on account of the sin of our first parent, and if a man be the executor of that sentence by Divine authority, he will be no murderer any more than God would be. Again Osee, by taking unto himself a wife of fornications, or an adulterous woman, was not guilty either of adultery or of fornication: because he took unto himself one who was his by command of God, Who is the Author of the institution of marriage.
Accordingly, therefore, the precepts of the decalogue, as to the essence of justice which they contain, are unchangeable: but as to any determination by application to individual actions--for instance, that this or that be murder, theft or adultery, or not--in this point they admit of change; sometimes by Divine authority alone, namely, in such matters as are exclusively of Divine institution, as marriage and the like; sometimes also by human authority, namely in such matters as are subject to human jurisdiction: for in this respect men stand in the place of God: and yet not in all respects.
Reply to Objection 4. This determination was an interpretation rather than a dispensation. For a man is not taken to break the Sabbath, if he does something necessary for human welfare; as Our Lord proves (Matthew 12:3, seqq.).
Source: Summa Theologica (New Advent)