Objection 1. It would seem that an oath is not voided by a condition of person or time. An oath, according to the Apostle (Hebrews 6:16), is employed for the purpose of confirmation. Now it is competent to anyone to confirm his assertion, and at any time. Therefore it would seem that an oath is not voided by a condition of person or time.
Objection 2. Further, to swear by God is more than to swear by the Gospels: wherefore Chrysostom [Hom. xliv in the Opus Imperfectum falsely ascribed to St. John Chrysostom] says: "If there is a reason for swearing, it seems a small thing to swear by God, but a great thing to swear by the Gospels. To those who think thus, it must be said: Nonsense! the Scriptures were made for God's sake, not God for the sake of the Scriptures." Now men of all conditions and at all times are wont to swear by God. Much more, therefore, is it lawful to swear by the Gospels.
Objection 3. Further, the same effect does not proceed from contrary causes, since contrary causes produce contrary effects. Now some are debarred from swearing on account of some personal defect; children, for instance, before the age of fourteen, and persons who have already committed perjury. Therefore it would seem that a person ought not to be debarred from swearing either on account of his dignity, as clerics, or on account of the solemnity of the time.
Objection 4. Further, in this world no living man is equal in dignity to an angel: for it is written (Matthew 11:11) that "he that is the lesser in the kingdom of heaven is greater than he," namely than John the Baptist, while yet living. Now an angel is competent to swear, for it is written (Apocalypse 10:6) that the angel "swore by Him that liveth for ever and ever." Therefore no man ought to be excused from swearing, on account of his dignity.
On the contrary, It is stated (II, qu. v, can. Si quis presbyter): "Let a priest be examined 'by his sacred consecration,' instead of being put on his oath": and (22, qu. v, can. Nullus): "Let no one in ecclesiastical orders dare to swear on the Holy Gospels to a layman."
I answer that, Two things are to be considered in an oath. One is on the part of God, whose testimony is invoked, and in this respect we should hold an oath in the greatest reverence. For this reason children before the age of puberty are debarred from taking oaths [Caus. XXII, qu. 5, can. Parvuli], and are not called upon to swear, because they have not yet attained the perfect use of reason, so as to be able to take a oath with due reverence. Perjurers also are debarred from taking an oath, because it is presumed from their antecedents that they will not treat an oath with the reverence due to it. For this same reason, in order that oaths might be treated with due reverence the law says (22, qu. v, can. Honestum): "It is becoming that he who ventures to swear on holy things should do so fasting, with all propriety and fear of God."
The other thing to be considered is on the part of the man, whose assertion is confirmed by oath. For a man's assertion needs no confirmation save because there is a doubt about it. Now it derogates from a person's dignity that one should doubt about the truth of what he says, wherefore "it becomes not persons of great dignity to swear." For this reason the law says (II, qu. v, can. Si quis presbyter) that "priests should not swear for trifling reasons." Nevertheless it is lawful for them to swear if there be need for it, or if great good may result therefrom. Especially is this the case in spiritual affairs, when moreover it is becoming that they should take oath on days of solemnity, since they ought then to devote themselves to spiritual matters. Nor should they on such occasions take oaths temporal matters, except perhaps in cases grave necessity.
Reply to Objection 1. Some are unable to confirm their own assertions on account of their own defect: and some there are whose words should be so certain that they need no confirmation.
Reply to Objection 2. The greater the thing sworn by, the holier and the more binding is the oath, considered in itself, as Augustine states (Ad Public., Ep. xlvii): and accordingly is a graver matter to swear by God than the Gospels. Yet the contrary may be the case on account of the manner of swearing for instance, an oath by the Gospels might be taken with deliberation and solemnity, and an oath by God frivolously and without deliberation.
Reply to Objection 3. Nothing prevents the same thing from arising out of contrary causes, by way of superabundance and defect. It is in this way that some are debarred from swearing, through being of so great authority that it is unbecoming for them to swear; while others are of such little authority that their oaths have no standing.
Reply to Objection 4. The angel's oath is adduced not on account of any defect in the angel, as though one ought not to credit his mere word, but in order to show that the statement made issues from God's infallible disposition. Thus too God is sometimes spoken of by Scripture as swearing, in order to express the immutability of His word, as the Apostle declares (Hebrews 6:17).
Source: Summa Theologica (New Advent)