Objection 1. It would seem that the active life is of greater merit than the contemplative. For merit implies relation to meed; and meed is due to labor, according to (1 Corinthians 3:8), "Every man shall receive his own reward according to his own labor." Now labor is ascribed to the active life, and rest to the contemplative life; for Gregory says (Hom. xiv in Ezech.): "Whosoever is converted to God must first of all sweat from labor, i.e. he must take Lia, that afterwards he may rest in the embraces of Rachel so as to see the principle." Therefore the active life is of greater merit than the contemplative.
Objection 2. Further, the contemplative life is a beginning of the happiness to come; wherefore Augustine commenting on John 21:22, "So I will have him to remain till I come," says (Tract. cxxiv in Joan.): "This may be expressed more clearly: Let perfect works follow Me conformed to the example of My passion, and let contemplation begun here remain until I come, that it may be perfected when I shall come." And Gregory says (Hom. xiv in Ezech.) that "contemplation begins here, so as to be perfected in our heavenly home." Now the life to come will be a state not of meriting but of receiving the reward of our merits. Therefore the contemplative life would seem to have less of the character of merit than the active, but more of the character of reward.
Objection 3. Further, Gregory says (Hom. xii in Ezech.) that "no sacrifice is more acceptable to God than zeal for souls." Now by the zeal for souls a man turns to the occupations of the active life. Therefore it would seem that the contemplative life is not of greater merit than the active.
On the contrary, Gregory says (Moral. vi, 37): "Great are the merits of the active life, but greater still those of the contemplative."
I answer that, As stated above (I-II, 114, 4), the root of merit is charity; and, while, as stated above (Question 25, Article 1), charity consists in the love of God and our neighbor, the love of God is by itself more meritorious than the love of our neighbor, as stated above (Question 27, Article 8). Wherefore that which pertains more directly to the love of God is generically more meritorious than that which pertains directly to the love of our neighbor for God's sake. Now the contemplative life pertains directly and immediately to the love of God; for Augustine says (De Civ. Dei xix, 19) that "the love of" the Divine "truth seeks a holy leisure," namely of the contemplative life, for it is that truth above all which the contemplative life seeks, as stated above (181, 4, ad 2). On the other hand, the active life is more directly concerned with the love of our neighbor, because it is "busy about much serving" (Luke 10:40). Wherefore the contemplative life is generically of greater merit than the active life. This is moreover asserted by Gregory (Hom. iii in Ezech.): "The contemplative life surpasses in merit the active life, because the latter labors under the stress of present work," by reason of the necessity of assisting our neighbor, "while the former with heartfelt relish has a foretaste of the coming rest," i.e. the contemplation of God.
Nevertheless it may happen that one man merits more by the works of the active life than another by the works of the contemplative life. For instance through excess of Divine love a man may now and then suffer separation from the sweetness of Divine contemplation for the time being, that God's will may be done and for His glory's sake. Thus the Apostle says (Romans 9:3): "I wished myself to be an anathema from Christ, for my brethren"; which words Chrysostom expounds as follows (De Compunct. i, 7 [Ad Demetr. de Compunct. Cordis.]): "His mind was so steeped in the love of Christ that, although he desired above all to be with Christ, he despised even this, because thus he pleased Christ."
Reply to Objection 1. External labor conduces to the increase of the accidental reward; but the increase of merit with regard to the essential reward consists chiefly in charity, whereof external labor borne for Christ's sake is a sign. Yet a much more expressive sign thereof is shown when a man, renouncing whatsoever pertains to this life, delights to occupy himself entirely with Divine contemplation.
Reply to Objection 2. In the state of future happiness man has arrived at perfection, wherefore there is no room for advancement by merit; and if there were, the merit would be more efficacious by reason of the greater charity. But in the present life contemplation is not without some imperfection, and can always become more perfect; wherefore it does not remove the idea of merit, but causes a yet greater merit on account of the practice of greater Divine charity.
Reply to Objection 3. A sacrifice is rendered to God spiritually when something is offered to Him; and of all man's goods, God specially accepts that of the human soul when it is offered to Him in sacrifice. Now a man ought to offer to God, in the first place, his soul, according to (Sirach 30:24), "Have pity on thy own soul, pleasing God"; in the second place, the souls of others, according to Apocalypse 22:17, "He that heareth, let him say: Come." And the more closely a man unites his own or another's soul to God, the more acceptable is his sacrifice to God; wherefore it is more acceptable to God that one apply one's own soul and the souls of others to contemplation than to action. Consequently the statement that "no sacrifice is more acceptable to God than zeal for souls," does not mean that the merit of the active life is preferable to the merit of the contemplative life, but that it is more meritorious to offer to God one's own soul and the souls of others, than any other external gifts.
Source: Summa Theologica (New Advent)