t is more
grievous to commit perjury in swearing by God than in swearing by the
Gospels.

Reply Obj. 3: Not every sin makes a man infamous in the eye of the
law. Wherefore, if a man who has sworn falsely in a declaratory oath
be not infamous in the eye of the law, but only when he has been so
declared by sentence in a court of law, it does not follow that he
has not sinned mortally. The reason why the law attaches infamy
rather to one who breaks a promissory oath taken solemnly is that he
still has it in his power after he has sworn to substantiate his
oath, which is not the case in a declaratory oath.
_______________________

FOURTH ARTICLE [II-II, Q. 98, Art. 4]

Whether He Sins Who Demands an Oath of a Perjurer?

Objection 1: It would seem that he who demands an oath of a perjurer
commits a sin. Either he knows that he swears truly, or he knows that
he swears falsely. If he knows him to swear truly, it is useless for
him to demand an oath: and if he believes him to swear falsely, for
his own part he leads him into sin. Therefore nowise seemingly should
one enjoin an oath on another person.

Obj. 2: Further, to receive an oath from a person is less than to
impose an oath on him. Now it would seem unlawful to receive an oath
from a person, especially if he swear falsely, because he would then
seem to consent in his sin. Much less therefore would it seem lawful
to impose an oath on one who swears falsely.

Obj. 3: Further, it is written (Lev. 5:1): "If anyone sin, and hear
the voice of one swearing falsely [*'Falsely' is not in the Vulgate],
and is a witness either because he himself hath seen, or is privy to
it: if he do not utter it, he shall bear his iniquity." Hence it
would seem that when a man knows another to be swearing falsely, he
is bound to denounce him. Therefore it is not lawful to demand an
oath of such a man.

Obj. 4: On the other hand, Just as it is a sin to swear falsely so is
it to swear by false gods. Yet it is lawful to take advantage of an
oath of one who has sworn by false gods, as Augustine says (ad
Public. Ep. xlvii). Therefore it is lawful to demand an oath from one
who swears falsely.

_I answer that,_ As regards a person who demands an oath from
another, a distinction would seem to be necessary. For either he
demands the oath on his own account and of his own accord, or he
demands it on account of the exigencies of a duty imposed on him. If
a man demands an oath on his own account