Objection 1. It would seem that he who has vowed to enter religion, is bound in perpetuity to remain in religion. For it is better not to enter religion than to leave after entering, according to (2 Peter 2:21), "It had been better for them not to have known the way of justice, than after they have known it to turn back," and (Luke 9:62), "No man putting his hand to the plough, and looking back, is fit for the kingdom of God." But he who bound himself by the vow to enter religion, is under the obligation to enter, as stated above (Article 3). Therefore he is also bound to remain for always.
Objection 2. Further, everyone is bound to avoid that which gives rise to scandal, and is a bad example to others. Now by leaving after entering religion a man gives a bad example and is an occasion of scandal to others, who are thereby withdrawn from entering or incited to leave. Therefore it seems that he who enters religion in order to fulfil a vow which he had previously taken, is bound to remain evermore.
Objection 3. Further, the vow to enter religion is accounted a perpetual vow: wherefore it is preferred to temporal vows, as stated above (3, ad 3; 88, 12, ad 1). But this would not be so if a person after vowing to enter religion were to enter with the intention of leaving. It seems, therefore, that he who vows to enter religion is bound also to remain in perpetuity.
On the contrary, The vow of religious profession, for the reason that it binds a man to remain in religion for evermore, has to be preceded by a year of probation; whereas this is not required before the simple vow whereby a man binds himself to enter religion. Therefore it seems that he who vows to enter religion is not for that reason bound to remain there in perpetuity.
I answer that, The obligation of a vow proceeds from the will: because "to vow is an act of the will" according to Augustine [Gloss of Peter Lombard on Psalm 75:12. Consequently the obligation of a vow extends as far as the will and intention of the person who takes the vow. Accordingly if in vowing he intend to bind himself not only to enter religion, but also to remain there evermore, he is bound to remain in perpetuity. If, on the other hand, he intend to bind himself to enter religion for the purpose of trial, while retaining the freedom to remain or not remain, it is clear that he is not bound to remain. If, however, in vowing he thought merely of entering religion, without thinking of being free to leave, or of remaining in perpetuity, it would seem that he is bound to enter religion according to the form prescribed by common law, which is that those who enter should be given a year's probation. Wherefore he is not bound to remain for ever.
Reply to Objection 1. It is better to enter religion with the purpose of making a trial than not to enter at all, because by so doing one disposes oneself to remain always. Nor is a person accounted to turn or to look back, save when he omits to do that which he engaged to do: else whoever does a good work for a time, would be unfit for the kingdom of God, unless he did it always, which is evidently false.
Reply to Objection 2. A man who has entered religion gives neither scandal nor bad example by leaving, especially if he do so for a reasonable motive; and if others are scandalized, it will be passive scandal on their part, and not active scandal on the part of the person leaving, since in doing so, he has done what was lawful, and expedient on account of some reasonable motive, such as sickness, weakness, and the like.
Reply to Objection 3. He who enters with the purpose of leaving forthwith, does not seem to fulfil his vow, since this was not his intention in vowing. Hence he must change that purpose, at least so as to wish to try whether it is good for him to remain in religion, but he is not bound to remain for evermore.
Source: Summa Theologica (New Advent)