Objection 1. It would seem that parish priests cannot lawfully enter religion. For Gregory says (Past. iii, 4) that "he who undertakes the cure of souls, receives an awful warning in the words: 'My son, if thou be surety for thy friend, thou hast engaged fast thy hand to a stranger'" (Proverbs 6:1); and he goes on to say, "because to be surety for a friend is to take charge of the soul of another on the surety of one's own behavior." Now he who is under an obligation to a man for a debt, cannot enter religion, unless he pay what he owes, if he can. Since then a priest is able to fulfil the cure of souls, to which obligation he has pledged his soul, it would seem unlawful for him to lay aside the cure of souls in order to enter religion.
Objection 2. Further, what is lawful to one is likewise lawful to all. But if all priests having cure of souls were to enter religion, the people would be left without a pastor's care, which would be unfitting. Therefore it seems that parish priests cannot lawfully enter religion.
Objection 3. Further, chief among the acts to which religious orders are directed are those whereby a man gives to others the fruit of his contemplation. Now such acts are competent to parish priests and archdeacons, whom it becomes by virtue of their office to preach and hear confessions. Therefore it would seem unlawful for a parish priest or archdeacon to pass over to religion.
On the contrary, It is said in the Decretals (XIX, qu. ii, cap. Duce sunt leges.): "If a man, while governing the people in his church under the bishop and leading a secular life, is inspired by the Holy Ghost to desire to work out his salvation in a monastery or under some canonical rule, even though his bishop withstand him, we authorize him to go freely."
I answer that, As stated above (3, ad 3; 88, 12, ad 1), the obligation of a perpetual vow stands before every other obligation. Now it belongs properly to bishops and religious to be bound by perpetual vow to devote themselves to the divine service [Cf. 184, 5], while parish priests and archdeacons are not, as bishops are, bound by a perpetual and solemn vow to retain the cure of souls. Wherefore bishops "cannot lay aside their bishopric for any pretext whatever, without the authority of the Roman Pontiff" (Extra, De Regular. et Transeunt. ad Relig., cap. Licet.): whereas archdeacons and parish priests are free to renounce in the hands of the bishop the cure entrusted to them, without the Pope's special permission, who alone can dispense from perpetual vows. Therefore it is evident that archdeacons and parish priests may lawfully enter religion.
Reply to Objection 1. Parish priests and archdeacons have bound themselves to the care of their subjects, as long as they retain their archdeaconry or parish, but they did not bind themselves to retain their archdeaconry or parish for ever.
Reply to Objection 2. As Jerome says (Contra Vigil.): "Although they," namely religious, "are sorely smitten by thy poisonous tongue, about whom you argue, saying; 'If all shut themselves up and live in solitude, who will go to church? who will convert worldlings? who will be able to urge sinners to virtue?' If this holds true, if all are fools with thee, who can be wise? Nor will virginity be commendable, for if all be virgins, and none marry, the human race will perish. Virtue is rare, and is not desired by many." It is therefore evident that this is a foolish alarm; thus might a man fear to draw water lest the river run dry. [St. Thomas gives no reply to the third objection, which is sufficiently solved in the body of the article.]
Source: Summa Theologica (New Advent)