Objection 1. It would seem that, in this life, perfection consists in the observance not of the commandments but of the counsels. For our Lord said (Matthew 19:21): "If thou wilt be perfect, go sell all [Vulgate: 'what'] thou hast, and give to the poor... and come, follow Me." Now this is a counsel. Therefore perfection regards the counsels and not the precepts.
Objection 2. Further, all are bound to the observance of the commandments, since this is necessary for salvation. Therefore, if the perfection of the Christian life consists in observing the commandments, it follows that perfection is necessary for salvation, and that all are bound thereto; and this is evidently false.
Objection 3. Further, the perfection of the Christian life is gauged according to charity, as stated above (Article 1). Now the perfection of charity, seemingly, does not consist in the observance of the commandments, since the perfection of charity is preceded both by its increase and by its beginning, as Augustine says (Super Canonic. Joan. Tract. ix). But the beginning of charity cannot precede the observance of the commandments, since according to John 14:2-3, "If any one love Me, he will keep My word." Therefore the perfection of life regards not the commandments but the counsels.
On the contrary, It is written (Deuteronomy 6:5): "Thou shalt love the Lord thy God with thy whole heart," and (Leviticus 19:18): "Thou shalt love thy neighbor [Vulgate: 'friend'] as thyself"; and these are the commandments of which our Lord said (Matthew 22:40): "On these two commandments dependeth the whole law and the prophets." Now the perfection of charity, in respect of which the Christian life is said to be perfect, consists in our loving God with our whole heart, and our neighbor as ourselves. Therefore it would seem that perfection consists in the observance of the precepts.
I answer that, Perfection is said to consist in a thing in two ways: in one way, primarily and essentially; in another, secondarily and accidentally. Primarily and essentially the perfection of the Christian life consists in charity, principally as to the love of God, secondarily as to the love of our neighbor, both of which are the matter of the chief commandments of the Divine law, as stated above. Now the love of God and of our neighbor is not commanded according to a measure, so that what is in excess of the measure be a matter of counsel. This is evident from the very form of the commandment, pointing, as it does, to perfection--for instance in the words, "Thou shalt love the Lord thy God with thy whole heart": since "the whole" is the same as "the perfect," according to the Philosopher (Phys. iii, 6), and in the words, "Thou shalt love thy neighbor as thyself," since every one loves himself most. The reason of this is that "the end of the commandment is charity," according to the Apostle (1 Timothy 1:5); and the end is not subject to a measure, but only such things as are directed to the end, as the Philosopher observes (Polit. i, 3); thus a physician does not measure the amount of his healing, but how much medicine or diet he shall employ for the purpose of healing. Consequently it is evident that perfection consists essentially in the observance of the commandments; wherefore Augustine says (De Perf. Justit. viii): "Why then should not this perfection be prescribed to man, although no man has it in this life?"
Secondarily and instrumentally, however, perfection consists in the observance of the counsels, all of which, like the commandments, are directed to charity; yet not in the same way. For the commandments, other than the precepts of charity, are directed to the removal of things contrary to charity, with which, namely, charity is incompatible, whereas the counsels are directed to the removal of things that hinder the act of charity, and yet are not contrary to charity, such as marriage, the occupation of worldly business, and so forth. Hence Augustine says (Enchiridion cxxi): "Whatever things God commands, for instance, 'Thou shalt not commit adultery,' and whatever are not commanded, yet suggested by a special counsel, for instance, 'It is good for a man not to touch a woman,' are then done aright when they are referred to the love of God, and of our neighbor for God's sake, both in this world and in the world to come." Hence it is that in the Conferences of the Fathers (Coll. i, cap. vii) the abbot Moses says: "Fastings, watchings, meditating on the Scriptures, penury and loss of all one's wealth, these are not perfection but means to perfection, since not in them does the school of perfection find its end, but through them it achieves its end," and he had already said that "we endeavor to ascend by these steps to the perfection of charity."
Reply to Objection 1. In this saying of our Lord something is indicated as being the way to perfection by the words, "Go, sell all thou hast, and give to the poor"; and something else is added wherein perfection consists, when He said, "And follow Me." Hence Jerome in his commentary on Matthew 19:27, says that "since it is not enough merely to leave, Peter added that which is perfect: 'And have followed Thee'"; and Ambrose, commenting on Luke 5:27, "Follow Me," says: "He commands him to follow, not with steps of the body, but with devotion of the soul, which is the effect of charity." Wherefore it is evident from the very way of speaking that the counsels are means of attaining to perfection, since it is thus expressed: "If thou wilt be perfect, go, sell," etc., as though He said: "By so doing thou shalt accomplish this end."
Reply to Objection 2. As Augustine says (De Perf. Justit. viii) "the perfection of charity is prescribed to man in this life, because one runs not right unless one knows whither to run. And how shall we know this if no commandment declares it to us?" And since that which is a matter of precept can be fulfilled variously, one does not break a commandment through not fulfilling it in the best way, but it is enough to fulfil it in any way whatever. Now the perfection of Divine love is a matter of precept for all without exception, so that even the perfection of heaven is not excepted from this precept, as Augustine says (De Perf. Justit. viii [Cf. De Spir. et Lit. XXXVI]), and one escapes transgressing the precept, in whatever measure one attains to the perfection of Divine love. The lowest degree of Divine love is to love nothing more than God, or contrary to God, or equally with God, and whoever fails from this degree of perfection nowise fulfils the precept. There is another degree of the Divine love, which cannot be fulfilled so long as we are on the way, as stated above (Article 2), and it is evident that to fail from this is not to be a transgressor of the precept; and in like manner one does not transgress the precept, if one does not attain to the intermediate degrees of perfection, provided one attain to the lowest.
Reply to Objection 3. Just as man has a certain perfection of his nature as soon as he is born, which perfection belongs to the very essence of his species, while there is another perfection which he acquires by growth, so again there is a perfection of charity which belongs to the very essence of charity, namely that man love God above all things, and love nothing contrary to God, while there is another perfection of charity even in this life, whereto a man attains by a kind of spiritual growth, for instance when a man refrains even from lawful things, in order more freely to give himself to the service of God.
Source: Summa Theologica (New Advent)