(a) The vow is invalid and illicit if the evil circumstance affects the thing promised itself, so that the fulfillment of the vow must be sinful, for example, when one promises to give an alms in order to seduce the recipient into sin or to build a church in order to gratify pride or spite. Similarly, invalid and illicit are vows made to obtain something evil (e.g., the vow of an alms in order to obtain success in a robbery) or to render thanksgiving for success in evil already done (e.g., the vow to give God half the booty taken in robbery); for such vows can not be fulfilled without the implicit protestation that God is the author of sin.
(b) The vow is valid but illicit if the evil circumstance affects only the act of vowing; for the thing promised is good and is to be performed properly, but the disposition of the vower is not free from sin as he makes his promise. It should be noted, however, that the evil circumstance does not always deprive the act of vowing of substantial goodness (see 78). Thus, if one vows to build a church and the sole motive for making the vow is the applause one will receive, the vow is substantially illicit; but if vainglory is only a secondary motive, the vow is substantially licit.
(c) The vow is valid and licit if the evil circumstance affects neither the act promised nor the act of vowing, both of these being good. Thus, it is lawful to vow an alms for every time one yields to a sinful habit. It is also lawful to vow an offering to God if one escapes unhurt from a duel, for such a vow does not ask God to bless the duel but to protect one’s life.
2205. Vows that Promise Something Indifferent.—(a) These vows are invalid if there is no circumstance to make the promise honorable to God (e.g., if one promises to save up a certain percentage of one’s earnings each month). The sin committed by those who vow necessary, impossible or indifferent things, does not seem to exceed venial fault _per se_, for the vow is illicit, not because its matter is evil and displeasing to God, but because it is not good and pleasing to Him. The sin seems to be one of levity rather than of irreverence.
(b) These vows are valid and lawful if there is a circumstance that makes the indifferent subject-matter honorable to God (e.g., if one vows to save up so much each month in order to practise frugality, or to set aside means for some charitable or pious cause).
2206. Meaning of a Better Good.—It is also necessary for validity of a vow that the thing promised be a better good; for this is the will of God, our sanctification (I Thess., iv. 3), and the vow is made to God.
(a) By the better good is not meant that which has no good superior to it, for then one could vow only the most excellent good, which is not true.
(b) By the better good, then, is understood that which is preferable to its contrary good (e.g., virginity is better than marriage), that which is absolutely or objectively preferable to its contradictory (e.g., it is better to give an alms than not to give one, it is better to keep the law of fasting than not to keep it), that which is relatively or subjectively better than its contradictory (e.g., it is better to marry than to commit fornication, or live in concubinage, or give scandal, or leave children illegitimate). Generally, however, it is not advisable to vow matrimony, for, even if the vow is not invalid, it seems to have little advantage. If a person thinks marriage is better for him, let him take the marriage vows or engage to marry the woman of his choice.
2207. Vows Invalidated by Promise of Lesser Good.—(a) The vow to do what is less pleasing to God (e.g., never to make a vow, never to embrace a counsel) is invalid _per se_. There may be cases, however, in which a vow of this kind would be better and therefore valid (e.g., when a person who is prone to making vows is bidden by the confessor to make no other vows without advice).
(b) The vow to do what may easily become less pleasing to God also seems to be invalid. Thus, if one were to make a vow to play no more games in order to give more time to prayer or to avoid temptations, the vow might later be a cause of spiritual harm, for at times it is more pleasing to God to take recreation than to abstain from it.