2221. When a Good Vow May Be Sinful.—A vow good in itself may be sinful or the occasion of sin on account of the dispositions of the vower.
(a) Thus, in taking vows one commits sin if one acts imprudently in not taking into consideration the circumstances. Hence, before making a vow one should consider carefully and consult one’s confessor or director or some other prudent man.
(b) After taking vows one commits sin by regrets, if these regrets include the desire not to comply with obligations (e.g., when one regrets having lived up to a vow in the past or intends not to live up to it in the future, or when one keeps the vow only from human motives and wishes one could violate it), or if they manifest ingratitude towards God (e.g., when one regrets without good reason that one ever took the vow). The sin committed by desire not to comply with obligation is mortal or venial according to the nature of the obligation; the sin of ingratitude is venial. There is no sin at all, it seems, if for a reasonable cause one regrets having made a vow or wishes there were no obligation to perform something of supererogation that one vowed, unless by such wishes one is exposed to temptations and the danger of sinning against the vow.
2222. Merit of Fulfilling a Vow that One Regrets.—If one regrets having made a vow but intends to keep it, is the good work performed better by reason of the vow?
(a) If the intention to keep the vow is prompted by a religious motive (e.g., the desire to please God or the fear to offend Him), the good work is more meritorious than if there were no vow, for it has the double value of an act of religion and of an act of some other virtue, of a good work done and of a vow to do it.
(b) If the intention to keep the vow is prompted by a human motive (e.g., the desire to please some human person or to secure some temporal benefit), the good work is not made more meritorious, but indeed is not meritorious at all, since not done for God’s sake.
2223. Who Can Make a Vow?—Every person living in the state of mortal existence, whether Catholic or non-Catholic, is able to take a vow, unless there is an impediment of natural or positive law.
(a) Natural law excludes vows made by those who are not masters of their own acts or who have not the use of reason; for, since a vow is a law which one knowingly places on oneself, it cannot be made by those who have not the right of disposition over their acts or who do not understand the meaning of the obligation. Hence, religious and other subject persons are restricted as to the right of making vows, while infants and insane persons are utterly incapable of making a vow.
(b) Positive law of the Church has laid down certain conditions for the validity of public vows (e.g., age, performance of the novitiate, etc.), and hence those persons in whom the conditions are not found are incapable of taking these vows. The power of binding and loosing has been given the Church, and the regulations on public vows exercise this power for the benefit of the Church as a whole and of the persons who take vows, As to private vows, it is a matter of dispute whether the Church has the power to appoint conditions for them, since they are internal acts (see 426), but it seems that no such conditions have ever been made.
2224. Twofold Dependence on the Will of Another.—Those who are not masters of their own acts are unable to vow on account of their subjection to or dependence on the will of another. There is a twofold dependence of this kind.