Reply Obj. 2: He who swears does indeed make use of his reverence or love for the person by whom he swears: he does not, however, direct his oath to the reverence or love of that person, but to something else that is necessary for the present life.
Reply Obj. 3: Even as a medicine is useful for healing, and yet, the stronger it is, the greater harm it does if it be taken unduly, so too an oath is useful indeed as a means of confirmation, yet the greater the reverence it demands the more dangerous it is, unless it be employed aright; for, as it is written (Ecclus. 23:13), "if he make it void," i.e. if he deceive his brother, "his sin shall be upon him: and if he dissemble it," by swearing falsely, and with dissimulation, "he offendeth double," (because, to wit, "pretended equity is a twofold iniquity," as Augustine [*Enarr. in Ps. lxiii, 7] declares): "and if he swear in vain," i.e. without due cause and necessity, "he shall not be justified." _______________________
SIXTH ARTICLE [II-II, Q. 89, Art. 6]
Whether It Is Lawful to Swear by Creatures?
Objection 1: It would seem that it is not lawful to swear by creatures. It is written (Matt. 5:34-36): "I say to you not to swear at all, neither by heaven . . . nor by the earth . . . nor by Jerusalem . . . nor by thy head": and Jerome, expounding these words, says: "Observe that the Saviour does not forbid swearing by God, but by heaven and earth," etc.
Obj. 2: Further, punishment is not due save for a fault. Now a punishment is appointed for one who swears by creatures: for it is written (22, qu. i, can. Clericum): "If a cleric swears by creatures he must be very severely rebuked: and if he shall persist in this vicious habit we wish that he be excommunicated." Therefore it is unlawful to swear by creatures.
Obj. 3: Further, an oath is an act of religion, as stated above (A. 4). But religious worship is not due to any creature, according to Rom. 1:23, 25. Therefore it is not lawful to swear by a creature.
On the contrary, Joseph swore "by the health of Pharaoh" (Gen. 42:16). Moreover it is customary to swear by the Gospel, by relics, and by the saints.
I answer that, As stated above (A. 1, ad 3), there are two kinds of oath. One is uttered as a simple contestation or calling God as witness: and this kind of oath, like faith, is based on God's truth. Now faith is essentially and chiefly about God Who is the very truth, and secondarily about creatures in which God's truth is reflected, as stated above (Q. 1, A. 1). In like manner an oath is chiefly referred to God Whose testimony is invoked; and secondarily an appeal by oath is made to certain creatures considered, not in themselves, but as reflecting the Divine truth. Thus we swear by the Gospel, i.e. by God Whose truth is made known in the Gospel; and by the saints who believed this truth and kept it.
The other way of swearing is by cursing and in this kind of oath a creature is adduced that the judgment of God may be wrought therein. Thus a man is wont to swear by his head, or by his son, or by some other thing that he loves, even as the Apostle swore (2 Cor. 1:23), saying: "I call God to witness upon my soul."