Objection 1. It would seem that it is not requisite for religious perfection that the three aforesaid, namely poverty, continence, and obedience, should come under a vow. For the school of perfection is founded on the principles laid down by our Lord. Now our Lord in formulating perfection (Matthew 19:21) said: "If thou wilt be perfect, go, sell all [Vulgate: 'what'] thou hast, and give to the poor," without any mention of a vow. Therefore it would seem that a vow is not necessary for the school of religion.
Objection 2. Further, a vow is a promise made to God, wherefore (Ecclesiastes 5:3) the wise man after saying: "If thou hast vowed anything to God, defer not to pay it," adds at once, "for an unfaithful and foolish promise displeaseth Him." But when a thing is being actually given there is no need for a promise. Therefore it suffices for religious perfection that one keep poverty, continence, and obedience without. vowing them.
Objection 3. Further, Augustine says (Ad Pollent., de Adult. Conjug. i, 14): "The services we render are more pleasing when we might lawfully not render them, yet do so out of love." Now it is lawful not to render a service which we have not vowed, whereas it is unlawful if we have vowed to render it. Therefore seemingly it is more pleasing to God to keep poverty, continence, and obedience without a vow. Therefore a vow is not requisite for religious perfection.
On the contrary, In the Old Law the Nazareans were consecrated by vow according to Numbers 6:2, "When a man or woman shall make a vow to be sanctified and will consecrate themselves to the Lord," etc. Now these were a figure of those "who attain the summit of perfection," as a gloss [Cf. Moral. ii] of Gregory states. Therefore a vow is requisite for religious perfection.
I answer that, It belongs to religious to be in the state of perfection, as shown above (Question 174, Article 5). Now the state of perfection requires an obligation to whatever belongs to perfection: and this obligation consists in binding oneself to God by means of a vow. But it is evident from what has been said (A3,4,5) that poverty, continence, and obedience belong to the perfection of the Christian life. Consequently the religious state requires that one be bound to these three by vow. Hence Gregory says (Hom. xx in Ezech.): "When a man vows to God all his possessions, all his life, all his knowledge, it is a holocaust"; and afterwards he says that this refers to those who renounce the present world.
Reply to Objection 1. Our Lord declared that it belongs to the perfection of life that a man follow Him, not anyhow, but in such a way as not to turn back. Wherefore He says again (Luke 9:62): "No man putting his hand to the plough, and looking back, is fit for the kingdom of God." And though some of His disciples went back, yet when our Lord asked (John 6:68-69), "Will you also go away?" Peter answered for the others: "Lord, to whom shall we go?" Hence Augustine says (De Consensu Ev. ii, 17) that "as Matthew and Mark relate, Peter and Andrew followed Him after drawing their boats on to the beach, not as though they purposed to return, but as following Him at His command." Now this unwavering following of Christ is made fast by a vow: wherefore a vow is requisite for religious perfection.
Reply to Objection 2. As Gregory says (Moral. ii) religious perfection requires that a man give "his whole life" to God. But a man cannot actually give God his whole life, because that life taken as a whole is not simultaneous but successive. Hence a man cannot give his whole life to God otherwise than by the obligation of a vow.
Reply to Objection 3. Among other services that we can lawfully give, is our liberty, which is dearer to man than aught else. Consequently when a man of his own accord deprives himself by vow of the liberty of abstaining from things pertaining to God's service, this is most acceptable to God. Hence Augustine says (Ep. cxxvii ad Paulin. et Arment.): "Repent not of thy vow; rejoice rather that thou canst no longer do lawfully, what thou mightest have done lawfully but to thy own cost. Happy the obligation that compels to better things."
Source: Summa Theologica (New Advent)