HKB
he fails to keep his oath, he is not guilty of perjury in this respect, since that which he swore to do was not a fit matter of an oath.
Reply Obj. 2: A person who swears not to enter religion, or not to give an alms, or the like, is guilty of perjury through lack of judgment. Hence when he does that which is best it is not an act of perjury, but contrary thereto: for the contrary of that which he is doing could not be a matter of an oath.
Reply Obj. 3: When one man swears or promises to do another's will, there is to be understood this requisite condition--that the thing commanded be lawful and virtuous, and not unbearable or immoderate.
Reply Obj. 4: An oath is a personal act, and so when a man becomes a citizen of a state, he is not bound, as by oath, to fulfil whatever the state has sworn to do. Yet he is bound by a kind of fidelity, the nature of which obligation is that he should take his share of the state's burdens if he takes a share of its goods.
The canon who swears to keep the statutes that have force in some particular "college" is not bound by his oath to keep any that may be made in the future, unless he intends to bind himself to keep all, past and future. Nevertheless he is bound to keep them by virtue of the statutes themselves, since they are possessed of coercive force, as stated above (I-II, Q. 96, A. 4).
THIRD ARTICLE [II-II, Q. 98, Art. 3]
Whether All Perjury Is a Mortal Sin?
Objection 1: It would seem that not all perjury is a mortal sin. It is laid down (Extra, De Jurejur., cap. Verum): "Referring to the question whether an oath is binding on those who have taken one in order to safeguard their life and possessions, we have no other mind than that which our predecessors the Roman Pontiffs are known to have had, and who absolved such persons from the obligations of their oath. Henceforth, that discretion may be observed, and in order to avoid occasions of perjury, let them not be told expressly not to keep their oath: but if they should not keep it, they are not for this reason to be punished as for a mortal sin." Therefore not all perjury is a mortal sin.
Obj. 2. Further, as Chrysostom [*Hom. xliv in the Opus Imperfectum on St. Matthew, falsely ascribed to St. John Chrysostom] says, "it is a greater thing to swear by God than by the Gospels." Now it is not always a mortal sin to swear by God to something false; for instance, if we were to employ such an oath in fun or by a slip of the tongue in the course of an ordinary conversation. Therefore neither is it always a mortal sin to break an oath that has been taken solemnly on the Gospels.
Obj. 3: Further, according to the Law a man incurs infamy through committing perjury (VI, qu. i, cap. Infames). Now it would seem that infamy is not incurred through any kind of perjury, as it is prescribed in the case of a declaratory oath violated by perjury [*Cap. Cum dilectus, de Ord. Cognit.]. Therefore, seemingly, not all perjury is a mortal sin.
On the contrary, Every sin that is contrary to a divine precept is a mortal sin. Now perjury is contrary to a divine precept, for it is written (Lev. 19:12): "Thou shalt not swear falsely by My name." Therefore it is a mortal sin.