Objection 1. It would seem that humility is the greatest of the virtues. For Chrysostom, expounding the story of the Pharisee and the publican (Luke 18), says [Eclog. hom. vii de Humil. Animi.] that "if humility is such a fleet runner even when hampered by sin that it overtakes the justice that is the companion of pride, whither will it not reach if you couple it with justice? It will stand among the angels by the judgment seat of God." Hence it is clear that humility is set above justice. Now justice is either the most exalted of all the virtues, or includes all virtues, according to the Philosopher (Ethic. v, 1). Therefore humility is the greatest of the virtues.
Objection 2. Further, Augustine says (De Verb. Dom., Serm. [S. 10, C): "Are you thinking of raising the great fabric of spirituality? Attend first of all to the foundation of humility." Now this would seem to imply that humility is the foundation of all virtue. Therefore apparently it is greater than the other virtues.
Objection 3. Further, the greater virtue deserves the greater reward. Now the greatest reward is due to humility, since "he that humbleth himself shall be exalted" (Luke 14:11). Therefore humility is the greatest of virtues.
Objection 4. Further, according to Augustine (De Vera Relig. 16), "Christ's whole life on earth was a lesson in moral conduct through the human nature which He assumed." Now He especially proposed His humility for our example, saying (Matthew 11:29): "Learn of Me, because I am meek and humble of heart." Moreover, Gregory says (Pastor. iii, 1) that the "lesson proposed to us in the mystery of our redemption is the humility of God." Therefore humility would seem to be the greatest of virtues.
On the contrary, Charity is set above all the virtues, according to Colossians 3:14, "Above all... things have charity." Therefore humility is not the greatest of virtues.
I answer that, The good of human virtue pertains to the order of reason: which order is considered chiefly in reference to the end: wherefore the theological virtues are the greatest because they have the last end for their object. Secondarily, however, it is considered in reference to the ordering of the means to the end. This ordinance, as to its essence, is in the reason itself from which it issues, but by participation it is in the appetite ordered by the reason; and this ordinance is the effect of justice, especially of legal justice. Now humility makes a man a good subject to ordinance of all kinds and in all matters; while every other virtue has this effect in some special matter. Therefore after the theological virtues, after the intellectual virtues which regard the reason itself, and after justice, especially legal justice, humility stands before all others.
Reply to Objection 1. Humility is not set before justice, but before that justice which is coupled with pride, and is no longer a virtue; even so, on the other hand, sin is pardoned through humility: for it is said of the publican (Luke 18:14) that through the merit of his humility "he went down into his house justified." Hence Chrysostom says [De incompr. Nat. Dei, Hom. v]: "Bring me a pair of two-horse chariots: in the one harness pride with justice, in the other sin with humility: and you will see that sin outrunning justice wins not by its own strength, but by that of humility: while you will see the other pair beaten, not by the weakness of justice, but by the weight and size of pride."
Reply to Objection 2. Just as the orderly assembly of virtues is, by reason of a certain likeness, compared to a building, so again that which is the first step in the acquisition of virtue is likened to the foundation, which is first laid before the rest of the building. Now the virtues are in truth infused by God. Wherefore the first step in the acquisition of virtue may be understood in two ways. First by way of removing obstacles: and thus humility holds the first place, inasmuch as it expels pride, which "God resisteth," and makes man submissive and ever open to receive the influx of Divine grace. Hence it is written (James 4:6): "God resisteth the proud, and giveth grace to the humble." On this sense humility is said to be the foundation of the spiritual edifice. Secondly, a thing is first among virtues directly, because it is the first step towards God. Now the first step towards God is by faith, according to Hebrews 11:6, "He that cometh to God must believe." On this sense faith is the foundation in a more excellent way than humility.
Reply to Objection 3. To him that despises earthly things, heavenly things are promised: thus heavenly treasures are promised to those who despise earthly riches, according to Matthew 6:19-20, "Lay not up to yourselves treasures on earth... but lay up to yourselves treasures in heaven." Likewise heavenly consolations are promised to those who despise worldly joys, according to Matthew 4:5, "Blessed are they that mourn, for they shall be comforted." On the same way spiritual uplifting is promised to humility, not that humility alone merits it, but because it is proper to it to despise earthly uplifting. Wherefore Augustine says (De Poenit. [Serm. cccli]): "Think not that he who humbles himself remains for ever abased, for it is written: 'He shall be exalted.' And do not imagine that his exaltation in men's eyes is effected by bodily uplifting."
Reply to Objection 4. The reason why Christ chiefly proposed humility to us, was because it especially removes the obstacle to man's spiritual welfare consisting in man's aiming at heavenly and spiritual things, in which he is hindered by striving to become great in earthly things. Hence our Lord, in order to remove an obstacle to our spiritual welfare, showed by giving an example of humility, that outward exaltation is to be despised. Thus humility is, as it were, a disposition to man's untrammeled access to spiritual and divine goods. Accordingly as perfection is greater than disposition, so charity, and other virtues whereby man approaches God directly, are greater than humility.
Source: Summa Theologica (New Advent)