Objection 1. It would seem that one who is bound by the vow to enter religion is not under an obligation of entering religion. For it is said in the Decretals (XVII, qu. ii, can. Consaldus): "Consaldus, a priest under pressure of sickness and emotional fervour, promised to become a monk. He did not, however, bind himself to a monastery or abbot; nor did he commit his promise to writing, but he renounced his benefice in the hands of a notary; and when he was restored to health he refused to become a monk." And afterwards it is added: "We adjudge and by apostolic authority we command that the aforesaid priest be admitted to his benefice and sacred duties, and that he be allowed to retain them in peace." Now this would not be if he were bound to enter religion. Therefore it would seem that one is not bound to keep one's vow of entering religion.
Objection 2. Further, no one is bound to do what is not in his power. Now it is not in a person's power to enter religion, since this depends on the consent of those whom he wishes to join. Therefore it would seem that a man is not obliged to fulfil the vow by which he bound himself to enter religion.
Objection 3. Further, a less useful vow cannot remit a more useful one. Now the fulfilment of a vow to enter religion might hinder the fulfilment of a vow to take up the cross in defense of the Holy Land; and the latter apparently is the more useful vow, since thereby a man obtains the forgiveness of his sins. Therefore it would seem that the vow by which a man has bound himself to enter religion is not necessarily to be fulfilled.
On the contrary, It is written (Ecclesiastes 5:3): "If thou hast vowed anything to God, defer not to pay it, for an unfaithful and foolish promise displeaseth him"; and a gloss on Psalm 75:12, "Vow ye, and pay to the Lord your God," says: "To vow depends on the will: but after the vow has been taken the fulfilment is of obligation."
I answer that, As stated above (Question 88, Article 1), when we were treating of vows, a vow is a promise made to God in matters concerning God. Now, as Gregory says in a letter to Boniface [Innoc. I, Epist. ii, Victricio Epo. Rotomag., cap. 14; Cf. can. Viduas: cause. xxvii, qu. 1: "If among men of good faith contracts are wont to be absolutely irrevocable, how much more shall the breaking of this promise given to God be deserving of punishment!" Therefore a man is under an obligation to fulfil what he has vowed, provided this be something pertaining to God.
Now it is evident that entrance into religion pertains very much to God, since thereby man devotes himself entirely to the divine service, as stated above (Question 186, Article 1). Hence it follows that he who binds himself to enter religion is under an obligation to enter religion according as he intends to bind himself by his vow: so that if he intend to bind himself absolutely, he is obliged to enter as soon as he can, through the cessation of a lawful impediment; whereas if he intend to bind himself to a certain fixed time, or under a certain fixed condition, he is bound to enter religion when the time comes or the condition is fulfilled.
Reply to Objection 1. This priest had made, not a solemn, but a simple vow. Hence he was not a monk in effect, so as to be bound by law to dwell in a monastery and renounce his cure. However, in the court of conscience one ought to advise him to renounce all and enter religion. Hence (Extra, De Voto et Voti Redemptione, cap. Per tuas) the Bishop of Grenoble, who had accepted the episcopate after vowing to enter religion, without having fulfilled his vow, is counseled that if "he wish to heal his conscience he should renounce the government of his see and pay his vows to the Most High."
Reply to Objection 2. As stated above (88, 3, ad 2), when we were treating of vows, he who has bound himself by vow to enter a certain religious order is bound to do what is in his power in order to be received in that order; and if he intend to bind himself simply to enter the religious life, if he be not admitted to one, he is bound to go to another; whereas if he intend to bind himself only to one particular order, he is bound only according to the measure of the obligation to which he has engaged himself.
Reply to Objection 3. The vow to enter religion being perpetual is greater than the vow of pilgrimage to the Holy Land, which is a temporal vow; and as Alexander III says (Extra, De Voto et Voti Redemptione, cap. Scripturae), "he who exchanges a temporary service for the perpetual service of religion is in no way guilty of breaking his vow."
Moreover it may be reasonably stated that also by entrance into religion a man obtains remission of all his sins. For if by giving alms a man may forthwith satisfy for his sins, according to (Daniel 4:24), "Redeem thou thy sins with alms," much more does it suffice to satisfy for all his sins that a man devote himself wholly to the divine service by entering religion, for this surpasses all manner of satisfaction, even that of public penance, according to the Decretals (XXXIII, qu. i, cap. Admonere) just as a holocaust exceeds a sacrifice, as Gregory declares (Hom. xx in Ezech.). Hence we read in the Lives of the Fathers (vi, 1) that by entering religion one receives the same grace as by being baptized. And yet even if one were not thereby absolved from all debt of punishment, nevertheless the entrance into religion is more profitable than a pilgrimage to the Holy Land, as regards the advancement in good, which is preferable to absolution from punishment.
Source: Summa Theologica (New Advent)