Objection 1. It would seem that none can be perfect in this life. For the Apostle says (1 Corinthians 13:1)0): "When that which is perfect is come, that which is in part shall be done away." Now in this life that which is in part is not done away; for in this life faith and hope, which are in part, remain. Therefore none can be perfect in this life.
Objection 2. Further, "The perfect is that which lacks nothing" (Phys. iii, 6). Now there is no one in this life who lacks nothing; for it is written (James 3:2): "In many things we all offend"; and (Psalm 138:16): "Thy eyes did see my imperfect being." Therefore none is perfect in this life.
Objection 3. Further, the perfection of the Christian life, as stated (1), relates to charity, which comprises the love of God and of our neighbor. Now, neither as to the love of God can one have perfect charity in this life, since according to Gregory (Hom. xiv in Ezech.) "the furnace of love which begins to burn here, will burn more fiercely when we see Him Whom we love"; nor as to the love of our neighbor, since in this life we cannot love all our neighbors actually, even though we love them habitually; and habitual love is imperfect. Therefore it seems that no one can be perfect in this life.
On the contrary, The Divine law does not prescribe the impossible. Yet it prescribes perfection according to Matthew 5:48, "Be you... perfect, as also your heavenly Father is perfect." Therefore seemingly one can be perfect in this life.
I answer that, As stated above (Article 1), the perfection of the Christian life consists in charity. Now perfection implies a certain universality because according to Phys. iii, 6, "the perfect is that which lacks nothing." Hence we may consider a threefold perfection. One is absolute, and answers to a totality not only on the part of the lover, but also on the part of the object loved, so that God be loved as much as He is lovable. Such perfection as this is not possible to any creature, but is competent to God alone, in Whom good is wholly and essentially.
Another perfection answers to an absolute totality on the part of the lover, so that the affective faculty always actually tends to God as much as it possibly can; and such perfection as this is not possible so long as we are on the way, but we shall have it in heaven.
The third perfection answers to a totality neither on the part of the object served, nor on the part of the lover as regards his always actually tending to God, but on the part of the lover as regards the removal of obstacles to the movement of love towards God, in which sense Augustine says (QQ. LXXXIII, qu. 36) that "carnal desire is the bane of charity; to have no carnal desires is the perfection of charity." Such perfection as this can be had in this life, and in two ways. First, by the removal from man's affections of all that is contrary to charity, such as mortal sin; and there can be no charity apart from this perfection, wherefore it is necessary for salvation. Secondly, by the removal from man's affections not only of whatever is contrary to charity, but also of whatever hinders the mind's affections from tending wholly to God. Charity is possible apart from this perfection, for instance in those who are beginners and in those who are proficient.
Reply to Objection 1. The Apostle is speaking there of heavenly perfection which is not possible to those who are on the way.
Reply to Objection 2. Those who are perfect in this life are said to "offend in many things" with regard to venial sins, which result from the weakness of the present life: and in this respect they have an "imperfect being" in comparison with the perfection of heaven.
Reply to Objection 3. As the conditions of the present life do not allow of a man always tending actually to God, so neither does it allow of his tending actually to each individual neighbor; but it suffices for him to tend to all in common and collectively, and to each individual habitually and according to the preparedness of his mind. Now in the love of our neighbor, as in the love of God we may observe a twofold perfection: one without which charity is impossible, and consisting in one's having in one's affections nothing that is contrary to the love of one's neighbor; and another without which it is possible to have charity. The latter perfection may be considered in three ways. First, as to the extent of love, through a man loving not only his friends and acquaintances but also strangers and even his enemies, for as Augustine says (Enchiridion lxxiii) this is a mark of the perfect children of God. Secondly, as to the intensity of love, which is shown by the things which man despises for his neighbor's sake, through his despising not only external goods for the sake of his neighbor, but also bodily hardships and even death, according to John 15:13, "Greater love than this no man hath, that a man lay down his life for his friends." Thirdly, as to the effect of love, so that a man will surrender not only temporal but also spiritual goods and even himself, for his neighbor's sake, according to the words of the Apostle (2 Corinthians 12:15), "But I most gladly will spend and be spent myself for your souls."
Source: Summa Theologica (New Advent)