Objection 1. It would seem that every religious is bound to keep all the counsels. For whoever professes a certain state of life is bound to observe whatever belongs to that state. Now each religious professes the state of perfection. Therefore every religious is bound to keep all the counsels that pertain to the state of perfection.
Objection 2. Further, Gregory says (Hom. xx in Ezech.) that "he who renounces this world, and does all the good he can, is like one who has gone out of Egypt and offers sacrifice in the wilderness." Now it belongs specially to religious to renounce the world. Therefore it belongs to them also to do all the good they can. and so it would seem that each of them is bound to fulfil all the counsels.
Objection 3. Further, if it is not requisite for the state of perfection to fulfil all the counsels, it would seem enough to fulfil some of them. But this is false, since some who lead a secular life fulfil some of the counsels, for instance those who observe continence. Therefore it would seem that every religious who is in the state of perfection is bound to fulfil whatever pertains to perfection: and such are the counsels.
On the contrary, one is not bound, unless one bind oneself, to do works of supererogation. But every religious does not bind himself to keep all the counsels, but to certain definite ones, some to some, others to others. Therefore all are not bound to keep all of them.
I answer that, A thing pertains to perfection in three ways. First, essentially, and thus, as stated above (Question 184, Article 3) the perfect observance of the precepts of charity belongs to perfection. Secondly, a thing belongs to perfection consequently: such are those things that result from the perfection of charity, for instance to bless them that curse you (Luke 6:27), and to keep counsels of a like kind, which though they be binding as regards the preparedness of the mind, so that one has to fulfil them when necessity requires; yet are sometimes fulfilled, without there being any necessity, through superabundance of charity. Thirdly, a thing belongs to perfection instrumentally and dispositively, as poverty, continence, abstinence, and the like.
Now it has been stated (1) that the perfection of charity is the end of the religious state. And the religious state is a school or exercise for the attainment of perfection, which men strive to reach by various practices, just as a physician may use various remedies in order to heal. But it is evident that for him who works for an end it is not necessary that he should already have attained the end, but it is requisite that he should by some means tend thereto. Hence he who enters the religious state is not bound to have perfect charity, but he is bound to tend to this, and use his endeavors to have perfect charity.
For the same reason he is not bound to fulfil those things that result from the perfection of charity, although he is bound to intend to fulfil them: against which intention he acts if he contemns them, wherefore he sins not by omitting them but by contempt of them.
In like manner he is not bound to observe all the practices whereby perfection may be attained, but only those which are definitely prescribed to him by the rule which he has professed.
Reply to Objection 1. He who enters religion does not make profession to be perfect, but he professes to endeavor to attain perfection; even as he who enters the schools does not profess to have knowledge, but to study in order to acquire knowledge. Wherefore as Augustine says (De Civ. Dei viii, 2), Pythagoras was unwilling to profess to be a wise man, but acknowledged himself, "a lover of wisdom." Hence a religious does not violate his profession if he be not perfect, but only if he despises to tend to perfection.
Reply to Objection 2. Just as, though all are bound to love God with their whole heart, yet there is a certain wholeness of perfection which cannot be omitted without sin, and another wholeness which can be omitted without sin (184, 2, ad 3), provided there be no contempt, as stated above (ad 1), so too, all, both religious and seculars, are bound, in a certain measure, to do whatever good they can, for to all without exception it is said (Ecclesiastes 9:1)0): "Whatsoever thy hand is able to do, do it earnestly." Yet there is a way of fulfilling this precept, so as to avoid sin, namely if one do what one can as required by the conditions of one's state of life: provided there be no contempt of doing better things, which contempt sets the mind against spiritual progress.
Reply to Objection 3. There are some counsels such that if they be omitted, man's whole life would be taken up with secular business; for instance if he have property of his own, or enter the married state, or do something of the kind that regards the essential vows of religion themselves; wherefore religious are bound to keep all such like counsels. Other counsels there are, however, about certain particular better actions, which can be omitted without one's life being taken up with secular actions; wherefore there is no need for religious to be bound to fulfil all of them.
Source: Summa Theologica (New Advent)