Objection 1. It would seem that mercy is the greatest of the virtues. For the worship of God seems a most virtuous act. But mercy is preferred before the worship of God, according to Hosea 6:6 and Matthew 12:7: "I have desired mercy and not sacrifice." Therefore mercy is the greatest virtue.
Objection 2. Further, on the words of 1 Timothy 4:8: "Godliness is profitable to all things," a gloss says: "The sum total of a Christian's rule of life consists in mercy and godliness." Now the Christian rule of life embraces every virtue. Therefore the sum total of all virtues is contained in mercy.
Objection 3. Further, "Virtue is that which makes its subject good," according to the Philosopher. Therefore the more a virtue makes a man like God, the better is that virtue: since man is the better for being more like God. Now this is chiefly the result of mercy, since of God is it said (Psalm 144:9) that "His tender mercies are over all His works," and (Luke 6:36) Our Lord said: "Be ye... merciful, as your Father also is merciful." Therefore mercy is the greatest of virtues.
On the contrary, The Apostle after saying (Colossians 3:12): "Put ye on... as the elect of God... the bowels of mercy," etc., adds (Colossians 3:14): "Above all things have charity." Therefore mercy is not the greatest of virtues.
I answer that, A virtue may take precedence of others in two ways: first, in itself; secondly, in comparison with its subject. On itself, mercy takes precedence of other virtues, for it belongs to mercy to be bountiful to others, and, what is more, to succor others in their wants, which pertains chiefly to one who stands above. Hence mercy is accounted as being proper to God: and therein His omnipotence is declared to be chiefly manifested [Collect, Tenth Sunday after Pentecost].
On the other hand, with regard to its subject, mercy is not the greatest virtue, unless that subject be greater than all others, surpassed by none and excelling all: since for him that has anyone above him it is better to be united to that which is above than to supply the defect of that which is beneath. ["The quality of mercy is not strained./'Tis mightiest in the mightiest: it becomes/The throned monarch better than his crown." Merchant of Venice, Act IV, Scene i.]. Hence, as regards man, who has God above him, charity which unites him to God, is greater than mercy, whereby he supplies the defects of his neighbor. But of all the virtues which relate to our neighbor, mercy is the greatest, even as its act surpasses all others, since it belongs to one who is higher and better to supply the defect of another, in so far as the latter is deficient.
Reply to Objection 1. We worship God by external sacrifices and gifts, not for His own profit, but for that of ourselves and our neighbor. For He needs not our sacrifices, but wishes them to be offered to Him, in order to arouse our devotion and to profit our neighbor. Hence mercy, whereby we supply others' defects is a sacrifice more acceptable to Him, as conducing more directly to our neighbor's well-being, according to Hebrews 13:16: "Do not forget to do good and to impart, for by such sacrifices God's favor is obtained."
Reply to Objection 2. The sum total of the Christian religion consists in mercy, as regards external works: but the inward love of charity, whereby we are united to God preponderates over both love and mercy for our neighbor.
Reply to Objection 3. Charity likens us to God by uniting us to Him in the bond of love: wherefore it surpasses mercy, which likens us to God as regards similarity of works.
Source: Summa Theologica (New Advent)