Objection 1. It would seem that it is not right to say that religious perfection consists in these three vows. For the perfection of life consists of inward rather than of outward acts, according to (Romans 14:17), "The Kingdom of God is not meat and drink, but justice and peace and joy in the Holy Ghost." Now the religious vow binds a man to things belonging to perfection. Therefore vows of inward actions, such as contemplation, love of God and our neighbor, and so forth, should pertain to the religious state, rather than the vows of poverty, continence, and obedience which refer to outward actions.
Objection 2. Further, the three aforesaid come under the religious vow, in so far as they belong to the practice of tending to perfection. But there are many other things that religious practice, such as abstinence, watchings, and the like. Therefore it would seem that these three vows are incorrectly described as pertaining to the state of perfection.
Objection 3. Further, by the vow of obedience a man is bound to do according to his superior's command whatever pertains to the practice of perfection. Therefore the vow of obedience suffices without the two other vows.
Objection 4. Further, external goods comprise not only riches but also honors. Therefore, if religious, by the vow of poverty, renounce earthly riches, there should be another vow whereby they may despise worldly honors.
On the contrary, It is stated (Extra, de Statu Monach., cap. Cum ad monasterium) that "the keeping of chastity and the renouncing of property are affixed to the monastic rule."
I answer that, The religious state may be considered in three ways. First, as being a practice of tending to the perfection of charity: secondly, as quieting the human mind from outward solicitude, according to (1 Corinthians 7:3)2: "I would have you to be without solicitude": thirdly, as a holocaust whereby a man offers himself and his possessions wholly to God; and in corresponding manner the religious state is constituted by these three vows.
First, as regards the practice of perfection a man is required to remove from himself whatever may hinder his affections from tending wholly to God, for it is in this that the perfection of charity consists. Such hindrances are of three kinds. First, the attachment to external goods, which is removed by the vow of poverty; secondly, the concupiscence of sensible pleasures, chief among which are venereal pleasures, and these are removed by the vow of continence; thirdly, the inordinateness of the human will, and this is removed by the vow of obedience. On like manner the disquiet of worldly solicitude is aroused in man in reference especially to three things. First, as regards the dispensing of external things, and this solicitude is removed from man by the vow of poverty; secondly, as regards the control of wife and children, which is cut away by the vow of continence; thirdly, as regards the disposal of one's own actions, which is eliminated by the vow of obedience, whereby a man commits himself to the disposal of another.
Again, "a holocaust is the offering to God of all that one has," according to Gregory (Hom. xx in Ezech.). Now man has a threefold good, according to the Philosopher (Ethic. i, 8). First, the good of external things, which he wholly offers to God by the vow of voluntary poverty: secondly, the good of his own body, and this good he offers to God especially by the vow of continence, whereby he renounces the greatest bodily pleasures. the third is the good of the soul, which man wholly offers to God by the vow of obedience, whereby he offers God his own will by which he makes use of all the powers and habits of the soul. Therefore the religious state is fittingly constituted by the three vows.
Reply to Objection 1. As stated above (Article 1), the end whereunto the religious vow is directed is the perfection of charity, since all the interior acts of virtue belong to charity as to their mother, according to (1 Corinthians 13:4), "Charity is patient, is kind," etc. Hence the interior acts of virtue, for instance humility, patience, and so forth, do not come under the religious vow, but this is directed to them as its end.
Reply to Objection 2. All other religious observances are directed to the three aforesaid principal vows; for if any of them are ordained for the purpose of procuring a livelihood, such as labor, questing, and so on, they are to be referred to poverty; for the safeguarding of which religious seek a livelihood by these means. Other observances whereby the body is chastised, such as watching, fasting, and the like, are directly ordained for the observance of the vow of continence. And such religious observances as regard human actions whereby a man is directed to the end of religion, namely the love of God and his neighbor (such as reading, prayer, visiting the sick, and the like), are comprised under the vow of obedience that applies to the will, which directs its actions to the end according to the ordering of another person. The distinction of habit belongs to all three vows, as a sign of being bound by them: wherefore the religious habit is given or blessed at the time of profession.
Reply to Objection 3. By obedience a man offers to God his will, to which though all human affairs are subject, yet some are subject to it alone in a special manner, namely human actions, since passions belong also to the sensitive appetite. Wherefore in order to restrain the passions of carnal pleasures and of external objects of appetite, which hinder the perfection of life, there was need for the vows of continence and poverty; but for the ordering of one's own actions accordingly as the state of perfection requires, there was need for the vow of obedience.
Reply to Objection 4. As the Philosopher says (Ethic. iv, 3), strictly and truly speaking honor is not due save to virtue. Since, however, external goods serve instrumentally for certain acts of virtue, the consequence is that a certain honor is given to their excellence especially by the common people who acknowledge none but outward excellence. Therefore since religious tend to the perfection of virtue it becomes them not to renounce the honor which God and all holy men accord to virtue, according to Psalm 138:17, "But to me Thy friends, O God, are made exceedingly honorable." On the other hand, they renounce the honor that is given to outward excellence, by the very fact that they withdraw from a worldly life: hence no special vow is needed for this.
Source: Summa Theologica (New Advent)