2256. Obligation Imposed by Promissory Oath.—An assertory oath imposes the obligation of telling the truth and of repairing any damage that results from the falsity or injustice of the declaration. In addition, a promissory oath binds one in virtue of religion to perform one’s promise; for, as said above (2249), an oath must have truth. Hence, Scripture bids those who have sworn to fulfill the promise (Matt., v. 33) not to make the word of no effect (Num, xxxi. 3). But the thing promised must be possible and lawful, or otherwise the oath lacks judgment or justice. (a) Thus, an impossible promise is not binding, for no one can oblige himself to perform what he cannot perform (cfr. 2201); (b) an unlawful promise is not binding, for no one can oblige himself to perform what he is bound not to do. Thus, an oath to revenge murder by murder is null, and sin is committed both in taking and in keeping it.
2257. Obligation Imposed by Negative Oaths.—The obligation of a sworn promise not to do what is better (e.g., not to take a vow), or to do what is vain and useless (e.g., an oath to count the steps one takes), depends on the circumstances. (a) If there are no rights of a third party involved, these oaths do not hold (e.g., he who swears not to vow acts laudably in disregarding the oath). For one may not call God to witness or be guarantee for that which is less pleasing to Him, or which in no way honors Him, and for which there is no claim on the part of a third person. (b) If there are rights of a third party involved, these oaths oblige one to give the third party what he is entitled to from the promise (e.g., a nurse who swears to remain with a sick person may not violate the oath by entering religion).
2258. Obligation of Oath Is Personal.—An oath added to a promise made to man and obligatory in justice is personal, and hence it binds the one who makes the oath, but not his heirs (see 2216).
2259. Interpretation of Promissory Oaths.—(a) An oath should be interpreted strictly, for the presumption is that the promisor intended to place upon himself the least possible burden. Thus, if a person swears to observe the statutes of a certain Congregation, it should be understood that he pledges himself to present, not to future statutes; if he swears not to gamble, the oath does not forbid games in which money is not played for. But if the promisor acts deceitfully, the oath is to be interpreted according to the intention of him who receives the promise (Canon 1321).
(b) An oath is always subject to the limitations and reservations which the nature of the case, law, or custom demands. Hence, even though an oath is made unconditionally, the following conditions are understood: “If fulfillment will be physically and morally possible,” “saving the rights of superiors,” “unless the other party renounces his right,” “unless the other party fails to keep his part of the agreement,” “unless there comes a notable change in conditions.” If the promisor explains beforehand to the promisee what he understands by the oath, he swears only in the sense thus set forth by him.
(c) An oath follows the nature and conditions of the act (e.g., resolution, promise, vow, contract) that it confirms, for the accessory follows the principal. Hence, if the act to which the oath is attached cannot be obligatory (e.g., an act detrimental to eternal salvation, or the public good, or the rights of a third party), the oath gives no strength to this act (Canon 1318); if the act is naturally invalid (e.g., a promise obtained through substantial fraud), the oath is also invalid; if the act does not become effective (e.g., a promise not accepted), neither does the oath become effective; if the act ceases to oblige (e.g., a promise of secrecy made for a time), the oath also ceases to oblige; if the act is not obligatory under grave sin, the oath is not obligatory under grave sin (e.g., if one swears to observe the statutes of a university, one is not bound to observe those that are commonly neglected, one commits no sin by transgressing those that are merely penal or optional, and one commits no grave sin by violating those that oblige under venial sin).
2260. Kind of Obligation Produced by a Valid Promissory Oath.—(a) The obligation is one of religion, because the significance of the oath is that it adds the duty of respect owed to God to the duty of fidelity owed to the promise. Men swear in order to make their promises more trustworthy through the sacredness of the oath. The violation of a promissory oath is, therefore, always a sin against religion. There are other sins added in some species of oath, namely, a second sin against religion in case of a sworn vow, a sin against justice and fidelity in case of a sworn contract, a second sin against religion and a sin against justice and fidelity in case of a sworn vow and contract (see 2246 a).
(b) The obligation, other things being equal, is less than that produced by a vow, because the vow binds in virtue of fidelity to God, but the oath only in virtue of respect. The obligation of fidelity seems to be stronger, because unfaithfulness always contains disrespect, but not vice versa. Moreover, in the case of a vow not only the fulfillment of the promise, but the thing promised itself is sacred, which is not true in the case of an oath. An assertory oath, however, seems to be more binding than a vow, because it is a greater injury to God to make Him the witness for falsehood than to break a promise made to Him.
2261. Degree of the Obligation of a Valid Promissory Oath.—(a) The obligation is grave, from the nature of an oath, because the virtue of religion is preeminent among the moral virtues (see 2146). There is no doubt that mortal sin is committed when one gives a sworn promise and has no intention to fulfill it, for this is perjury (see 2250 b); and also when one unjustly refuses to live up to an important engagement made under oath, for this is irreligion and injustice in a serious matter. The remarks on grave matter in vows (2211) apply here, but, since the vow obliges more strictly, a somewhat greater amount is needed for serious matter in violation of an oath.
(b) The obligation may be light on account of the smallness of the matter involved. Even a vow, which is more binding than an oath, may be of venial obligation in this way (see 2211). A person who makes a promise under oath, fully intending to keep the promise, but who later changes his mind without sufficient reason, does not show disrespect to God, since when the oath was made he intended to abide by it, and does not seriously injure his neighbor, since, as we suppose, the matter of the oath is small. The sin, therefore, is one of inconstancy or levity, and, if there is disrespect, it is slight. Thus, if a person who had sworn to drink no more wine took a drop now and then, these transgressions would be only venial. Some authors, however, believe that every unfaithfulness to a promissory oath, no matter how small the subject-matter, is a grave sin, because perjury is committed by the breach of promise. This is commonly denied, because the meaning of a promissory oath is that God is called on to witness the truth of a present intention and the obligation (great or small) of a future performance.