How great the force of an oath

Among all peoples, and in every age, the force of an oath regarding promises, agreements, and contracts has always been very great.…Hence at no time has it failed to be believed that a severe punishment awaits perjurers. [It was sometimes believed] that even posterity might pay the penalty for the sins of ancestors, an opinion that was held only with regard to the worst crimes; and the wish also, without the act, might bring punishment on itself.

Let us now see whence the force of an oath arises, and to what point it extends.

Requirement of a deliberate intention

First, the statement which we made about promises and contracts, that a mind possessed of reason and a deliberate intention are requisite, is in place here.

If any one has been willing to swear but unwilling to bind himself, he is nonetheless bound, because the obligation is inseparable from the oath and is a necessary result of it.

That the words of an oath are binding in the sense in which it is believed

But if anyone has deliberately uttered the words of an oath, yet without the intention of swearing, some writers state that he is not bound, but yet that he sins by swearing rashly. It is, however, nearer the truth to say that he is bound to make true the words which he has called God to witness. For that act, which is binding in itself, proceeded from a deliberate intention.