As appears from what has been said above (A. 1), he
that swears calls God to witness in confirmation of what he says. Now
nothing is confirmed save by what is more certain and more powerful.
Therefore in the very fact that a man swears by God, he acknowledges
God to be more powerful, by reason of His unfailing truth and His
universal knowledge; and thus in a way he shows reverence to God. For
this reason the Apostle says (Heb. 6:16) that "men swear by one
greater than themselves," and Jerome commenting on Matt. 5:34, says
that "he who swears either reveres or loves the person by whom he
swears." The Philosopher, too, states (Metaph. i, 3) that "to swear
is to give very great honor." Now to show reverence to God belongs to
religion or latria. Wherefore it is evident that an oath is an act of
religion or latria.

Reply Obj. 1: Two things may be observed in an oath. The witness
adduced, and this is Divine: and the thing witnessed to, or that
which makes it necessary to call the witness, and this is human.
Accordingly an oath belongs to religion by reason of the former, and
not of the latter.

Reply Obj. 2: In the very fact that a man takes God as witness by way
of an oath, he acknowledges Him to be greater: and this pertains to
the reverence and honor of God, so that he offers something to God,
namely, reverence and honor.

Reply Obj. 3: Whatsoever we do, we should do it in honor of God:
wherefore there is no hindrance, if by intending to assure a man, we
show reverence to God. For we ought so to perform our actions in
God's honor that they may conduce to our neighbor's good, since God
also works for His own glory and for our good.
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FIFTH ARTICLE [II-II, Q. 89, Art. 5]

Whether Oaths Are Desirable and to Be Used Frequently As Something
Useful and Good?

Objection 1: It would seem that oaths are desirable and to be used
frequently as something useful and good. Just as a vow is an act of
religion, so is an oath. Now it is commendable and more meritorious
to do a thing by vow, because a vow is an act of religion, as stated
above (Q. 88, A. 5). Therefore for the same reason, to do or say a
thing with an oath is more commendable, and consequently oaths are
desirable as being good essentially.

Obj. 2: Further, Jerome, commenting on Matt. 5:34, says that "he who
swears either reveres or loves the person by whom he swears." Now
reverence and love of God are desirable as something good
essential