Catechism of the Council of Trent: No Misuse of God's Name
Conditions Of A Lawful Oath
Although to constitute an oath it is sufficient to call God to witness, yet to constitute a holy and just oath many other conditions are required, which should be carefully explained. These, as St Jerome observes, are briefly enumerated in the words of Jeremias: Thou shalt swear: as the Lord liveth, in truth and in judgment and in justice, words which briefly sum up all the conditions that constitute the perfection of an oath, namely, truth, judgment, justice.
First Condition: Truth
Truth, then, holds the first place in an oath. What is asserted must be true and he who swears must believe what he swears to be true, being influenced not by rash judgment or mere conjecture, but by solid reasons.
Truth is a condition not less necessary in a promissory than in an affirmatory oath. He who promises must be disposed to perform and fulfil his promise at the appointed time As no conscientious man will promise to do what he considers opposed to the most holy Commandments and will of God; so, having promised and sworn to do what is lawful, he will never fail to adhere to his engagement, unless, perhaps by a change of circumstances it should happen that, if he wished to keep faith and observe his promises, he must incur the displeasure and enmity of God. That truth is necessary to an oath David also declares in these words: He that sweareth to his neighbour, and deceiveth not.
Second Condition: Judgment
The second condition of an oath is judgment. An oath is not to be taken rashly and inconsiderately, but after deliberation and reflection. When about to take an oath, therefore, one should first consider whether he is obliged to take it, and should weigh well the whole case, reflecting whether it seems to call for an oath. Many other circumstances of time, place, etc., are also to be taken into consideration; and one should not be influenced by love or hatred, or any other passion, but by the nature and necessity of the case.
Unless this careful consideration and reflection precede, an oath must be rash and hasty; and of this character are the irreligious affirmations of those, who, on the most unimportant and trifling occasions, swear without thought or reason from the influence of bad habit alone. This we see practiced daily everywhere among buyers and sellers. The latter, to sell at the highest price, the former to purchase at the cheapest rate, make no scruple to strengthen with an oath their praise or dispraise of the goods on sale.
Since, therefore, judgment and prudence are necessary, and since children are not able, on account of their tender years, to understand and judge accurately, Pope St. Cornelius decreed that an oath should not be administered to children before puberty, that is, before their fourteenth year.
Third Condition: Justice
The last condition (of an oath) is justice, which is especially requisite in promissory oaths. Hence, if a person swear to do what is unjust or unlawful, he sins by taking the oath, and adds sin to sin by executing his promise. Of this the Gospel supplies an example. King Herod, bound by a rash oath, gave to a dancing girl the head of John the Baptist as a reward for her dancing. Such was also the oath taken by the Jews, who, as we read in the Acts of the Apostles, bound themselves by oath not to eat, until they had killed Paul.
Lawfulness Of Oaths
These explanations having been given, there can be no doubt that they who observe the above conditions and who guard their oaths with these qualities as with bulwarks, may swear with a safe conscience.
This is easily established by many proofs. For the law of God, which is pure and holy, commands: Thou shalt fear the Lord thy God, and shalt serve him only, and thou shalt swear by his name. All they, writes David, shall be praised that swear by him.
The Scriptures also inform us that the most holy Apostles, the lights of the Church, sometimes made use of oaths, as appears from the Epistles of the Apostle.
Even the Angels sometimes swear. The angel, writes St. John in the Apocalypse, swore by him who lives for ever.
Nay, God Himself, the Lord of Angels, swears, and, as we read in many passages of the Old Testament, has confirmed His promises with an oath. This He did to Abraham and to David. Of the oath sworn by God David says: The Lord hath sworn, and he will not repent: thou art a priest for ever according to the order of Melchisedech.
In fact, if we consider the whole matter attentively, and examine the origin and purpose of an oath, it can be no difficult matter to explain the reasons why it is a laudable act.
An oath has its origin in faith, by which men believe God to be the author of all truth, who can never deceive others nor be deceived, to whose eyes all things are naked and open, who, in fine, superintends all human affairs with an admirable providence, and governs the world. Filled with this faith we appeal to God as a witness of the truth, as a witness whom it would be wicked and impious to distrust.
With regard to the end of an oath, its scope and intent is to establish the justice and innocence of man, and to terminate disputes and contests. This is the doctrine of the Apostle in his Epistle to the Hebrews.
An Objection Against Oaths
Nor does this doctrine at all clash with these words of the Redeemer, recorded in St. Matthew: You have heard that it was said to them of old: "Thou shalt not foreswear thyself, but thou shalt perform thy oaths to the Lord"; but I say to you not to swear at all; neither by heaven, for it is the throne of God; neither by the earth, for it is his footstool; nor by Jerusalem, for it is the city of the great king; neither shalt thou swear by thy head, because thou canst not make one hair white or black. But let your speech be "yea, yea"; "no, no"; and that which is over and above these is of evil.
It cannot be asserted that these words condemn oaths universally and under all circumstances, since we have already seen that the Apostles and our Lord Himself made frequent use of them. The object of our Lord was rather to reprove the perverse opinion of the Jews, who had persuaded themselves that the only thing to be avoided in an oath was a lie. Hence in matters the most trivial and unimportant they did not hesitate to make frequent use of oaths, and to exact them from others. This practice the Redeemer condemns and reprobates, and teaches that an oath is never to be taken unless necessity require it. For oaths have been instituted on account of human frailty. They are really the outcome of evil, being a sign either of the inconstancy of him who takes them, or of the obstinacy of him who refuses to believe without them. However, an oath can be justified by necessity.
When our Lord says: Let your speech be "yea, yea"; "no, no," He evidently forbids the habit of swearing in familiar conversation and on trivial matters. He therefore admonishes us particularly against being too ready and willing to swear; and this should be carefully explained and impressed on the minds of the faithful. That countless evils grow out of the unrestrained habit of swearing is proved by the evidence of Scripture, and the testimony of the most holy Fathers. Thus we read in Ecclesiasticus: Let not thy mouth be accustomed to swearing, for in it there are many falls; and again: A man that sweareth much shall be filled with iniquity, and a scourge shall not depart from his house. In the works of St. Basil and St. Augustine against lying, much more can be found on this subject.
Negative Part of this Commandment
So far we have considered what this Commandment requires. It now remains to speak of what it prohibits; namely, to take the name of God in vain. It is clear that he who swears rashly and without deliberation commits a grave sin. That this is a most serious sin is declared by the words: Thou shalt not take the name of thy God in vain, which seem to assign the reason why this crime is so wicked and heinous; namely, that it derogates from the majesty of Him whom we profess to recognise as our Lord and our God. This Commandment, therefore, forbids to swear falsely, because he who does not shrink from so great a crime as to appeal to God to witness falsehood, offers a grievous Injury to God, charging Him either with ignorance, as though the truth of any matter could be unknown to Him, or with malice and dishonesty, as though God could bear testimony to falsehood.
Various Ways In Which God's Name Is Dishonoured: False Oaths
Among false swearers are to be numbered not only those who affirm as true what they know to be false, but also those who swear to what is really true, believing it to be false. For since the essence of a lie consists in speaking contrary to one's belief and conviction, these persons are evidently guilty of a lie, and of perjury.
On the same principle, he who swears to that which he thinks to be true, but which is really false, also incurs the guilt of perjury, unless he has used proper care and diligence to arrive at a full knowledge of the matter. Although he swears according to his belief, he nevertheless sins against this Commandment.
Again, he who binds himself by oath to the performance of anything, not intending to fulfil his promise, or, having had the intention, neglect its performance, is guilty of the same sin. This equally applies to those who, having bound themselves to God by vow, neglect its fulfilment.