h an
oath wherein God is called to witness: because it suffices for the
truth of an assertion, that a person say what he proposes to do,
since it is already true in its cause, namely, the purpose of the
doer. But an oath should not be employed, save in a matter about
which one is firmly certain: and, consequently, if a man employ an
oath, he is bound, as far as he can, to make true what he has sworn,
through reverence of the Divine witness invoked, unless it leads to
an evil result, as stated.

Reply Obj. 2: An oath may lead to an evil result in two ways. First,
because from the very outset it has an evil result, either through
being evil of its very nature (as, if a man were to swear to commit
adultery), or through being a hindrance to a greater good, as if a
man were to swear not to enter religion, or not to become a cleric,
or that he would not accept a prelacy, supposing it would be
expedient for him to accept, or in similar cases. For oaths of this
kind are unlawful from the outset: yet with a difference: because if
a man swear to commit a sin, he sinned in swearing, and sins in
keeping his oath: whereas if a man swear not to perform a greater
good, which he is not bound to do withal, he sins indeed in swearing
(through placing an obstacle to the Holy Ghost, Who is the inspirer
of good purposes), yet he does not sin in keeping his oath, though he
does much better if he does not keep it.

Secondly, an oath leads to an evil result through some new and
unforeseen emergency. An instance is the oath of Herod, who swore to
the damsel, who danced before him, that he would give her what she
would ask of him. For this oath could be lawful from the outset,
supposing it to have the requisite conditions, namely, that the
damsel asked what it was right to grant, but the fulfilment of the
oath was unlawful. Hence Ambrose says (De Officiis i, 50): "Sometimes
it is wrong to fulfil a promise, and to keep an oath; as Herod, who
granted the slaying of John, rather than refuse what he had promised."

Reply Obj. 3: There is a twofold obligation in the oath which
a man takes under compulsion: one, whereby he is beholden to the
person to whom he promises something; and this obligation is cancelled
by the compulsion, because he that used force deserves that the
promise made to him should not be kept. The other is an obligation
whereby a man is beholden to God, in virtue of which he is bound to
fulfil what he has promised in His name.