(b) It is not lawful to take an oath that lacks a necessary quality. Here we should note an important difference between an oath and other acts of religion, such as vows. An oath is not desirable for its own sake, since it is occasioned by human weakness and unreliability; hence, like medicine and other necessities occasioned by evil, it should be used only in serious need and sparingly. A vow or other act of religion, on the contrary, originates from the desire to honor God, even apart from necessity, and hence it may be used oftener. This explains why Scripture forbids the habit of swearing (Ecclus., xxiii. 9; Matt., v. 33; James, v. 12); but it is a wrong interpretation of these texts that sees in them an absolute prohibition of oaths. From the context and other passages it is clear that the Scriptures just cited reprove the Pharisees who taught that promiscuous swearing was lawful, provided only the matter was true or the Divine Name was not used, and also those persons who delighted to swear on all occasions.
2249. Necessary Qualities of a Lawful Oath.—The necessary qualities that should accompany an oath are expressed in Jeremias, iv. 2: “And thou shalt swear, ‘As the Lord liveth,’ in truth, and in judgment and in justice.” Judgment refers to the good dispositions of the person who swears, truth and justice to the righteousness of the cause for which he swears.
(a) Thus, an oath should have judgment; that is, the person who swears should do so only from serious necessity, with faith and devotion, and in a manner respectful to God whom he invokes. An oath that lacks judgment is called incautious or disrespectful, as when one swears about a trivial matter or swears jokingly.
(b) An oath should have truth; that is, one should not swear except to that which one believes to be true, after reasonable diligence has been used in seeking for the truth. An oath that lacks truth is called false or perjured, as when one swears to what one knows or believes to be false, or promises what one does not intend to fulfill, or swears that one is certain when one has only opinion, or swears with a purely mental reservation, or swears after insufficient investigation of a matter.
(c) An oath should have justice; that is, one should not promise what one has no right to promise (e.g., to tell a lie), and one should not say what one has no right to say (e.g., what is defamatory). The matter of the oath, then, both as to its object and its circumstances, must be good, even though one is swearing truthfully and respectfully. An oath that lacks justice is called a wicked oath, as when one promises under oath to commit murder, or not to follow what is of counsel, or swears about a real fact in such a way as to do unnecessary harm to another person or to boast about one’s own crimes.
2250. Sinful Oaths.—(a) An incautious or disrespectful oath is from its nature only a venial sin, since its malice consists, not in any direct injury to the divine truth or other attribute, but only in levity of mind; and, moreover, it is not opposed to the purpose of an oath, which is to confirm the truth. But accidentally it may be a serious sin on account of the scandal it gives (e.g., when a person of standing swears without necessity), or on account of the danger to which it exposes the swearer (e.g., when one swears habitually and is thereby put in the occasion of swearing falsely or unjustly). On account of the evils of familiarity, etc., to which habitual swearing leads, Our Lord warn us to be content as a rule to support the truth with simple assertion or denial (Matt., v. 2). At least for ordinary, daily communications the word of a Christian or honest man ought to be sufficient without his oath.
(b) A lying or perjured oath is from the nature of the act always (see 172) a mortal sin, since it consists essentially in contempt for God and disrespect for His attributes. The perjurer dares to ask God to be an accomplice in a lie, or else supposes that God can be deceived. Hence, only by reason of the imperfection of the act can perjury ever escape the guilt of mortal sin, as when one commits perjury without sufficient reflection on or full consent to the oath or to its falsity. Pope Innocent XI condemned the doctrine that perjury is only a light sin (Denzinger, n. 1174). In Canon Law those who perjure themselves are debarred from acting as witnesses or giving expert testimony, and are subject to penalties at the discretion of the Ordinary (see Canons 1757, 1795, 2323). In American civil law perjury is a false oath given before a tribunal and is a crime against public justice, while subornation of perjury and false oaths given on private occasions are also crimes or punishable offenses.
(c) A wicked oath, even though the thing sworn to be true and the oath be given only after consideration and in a respectful manner, is a sin against religion and any other virtue it offends. The sin committed by reason of the oath is from its nature mortal according to some, since the swearer gravely insults God by asking Him to become a partaker in sin and by turning into an instrument of sin what should be an act of religion; others hold that the sin is only venial, since it is not serious disrespect to ask God to witness the truth of what is true; others again make the gravity of the sin depend on the wickedness of the matter or circumstances. This wickedness committed by reason of the statement or promise is venial or mortal according to the case. Thus, there is grave injustice in revealing a fact seriously detrimental to another and which one is bound to keep as confidential; there is venial scandal in swearing in order to lead another person into a slight fault of detraction; there is a grave sin of impurity in promising to commit adultery; there is a light sin of theft in promising to steal a small sum of money. Finally, others hold that the oath is mortally sinful when it furthers a grave sin (e.g., an oath confirming serious detraction), and that it is venial in other cases (e.g., an oath confirming a boast about past mortal sins).
2251. Mental Reservation in an Oath.-(a) Strict mental reservation (i.e., the internal restriction of one’s words so that the listener cannot gather the true meaning, as when one says one has seen Rome, meaning a picture of Rome) is a lie, and hence cannot be used in an oath without perjury. See propositions condemned by Innocent XI (Denzinger, n. 1176).
(b) Wide mental reservation (i.e., the internal restriction of one’s words that may be gathered by the listener from circumstances, as when a servant says his master is not at home, meaning that he cannot be seen) is lawful only when there is some reason of justice or charity that demands it. Hence, it is a mortal sin to swear with this kind of reservation when the questioner has the right to know the truth; it is no sin at all when the questioner has no right to question and mental reservation is the only escape from a serious evil. If the questioner has no right to demand an oath and the deponent has no right to use mental reservation (e.g., when the oath is only private and not concerned with contract or other important matter), the sinfulness of a mental reservation is a matter of dispute. Some think the oath is mortally sinful, because it is gravely irreverent to God to call on Him to witness testimony meant to deceive. Others think the oath is only venially sinful, because the offense is not against truth or justice, but only against judgment or discretion.