HKB
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 (A. 3, ad 3; Q. 88, A. 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. Q. 184, A. 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 Obj. 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 Obj. 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.]
EIGHTH ARTICLE [II-II, Q. 189, Art. 8]
Whether It Is Lawful to Pass from One Religious Order to Another?
Objection 1: It seems unlawful to pass from one religious order to another, even a stricter one. For the Apostle says (Heb. 10:25): "Not forsaking our assembly, as some are accustomed"; and a gloss observes: "Those namely who yield through fear of persecution, or who presuming on themselves withdraw from the company of sinners or of the imperfect, that they may appear to be righteous." Now those who pass from one religious order to another more perfect one would seem to do this. Therefore this is seemingly unlawful.
Obj. 2: Further, the profession of monks is stricter than that of canons regular (Extra, De Statu Monach. et Canonic. Reg., cap. Quod Dei timorem). But it is unlawful for anyone to pass from the state of canon regular to the monastic state. For it is said in the Decretals (XIX, qu. iii, can. Mandamus): "We ordain and without any exception forbid any professed canon regular to become a monk, unless (which God forbid) he have fallen into public sin." Therefore it would seem unlawful for anyone to pass from one religious order to another of higher rank.