Objection 1. It seems that mercy cannot be attributed to God. For mercy is a kind of sorrow, as Damascene says (De Fide Orth. ii, 14). But there is no sorrow in God; and therefore there is no mercy in Him.
Objection 2. Further, mercy is a relaxation of justice. But God cannot remit what appertains to His justice. For it is said (2 Timothy 2:13): "If we believe not, He continueth faithful: He cannot deny Himself." But He would deny Himself, as a gloss says, if He should deny His words. Therefore mercy is not becoming to God.
On the contrary, it is said (Psalm 110:4): "He is a merciful and gracious Lord."
I answer that, Mercy is especially to be attributed to God, as seen in its effect, but not as an affection of passion. In proof of which it must be considered that a person is said to be merciful [misericors], as being, so to speak, sorrowful at heart [miserum cor]; being affected with sorrow at the misery of another as though it were his own. Hence it follows that he endeavors to dispel the misery of this other, as if it were his; and this is the effect of mercy. To sorrow, therefore, over the misery of others belongs not to God; but it does most properly belong to Him to dispel that misery, whatever be the defect we call by that name. Now defects are not removed, except by the perfection of some kind of goodness; and the primary source of goodness is God, as shown above (Question 6, Article 4). It must, however, be considered that to bestow perfections appertains not only to the divine goodness, but also to His justice, liberality, and mercy; yet under different aspects. The communicating of perfections, absolutely considered, appertains to goodness, as shown above (6, 1, 4); in so far as perfections are given to things in proportion, the bestowal of them belongs to justice, as has been already said (1); in so far as God does not bestow them for His own use, but only on account of His goodness, it belongs to liberality; in so far as perfections given to things by God expel defects, it belongs to mercy.
Reply to Objection 1. This argument is based on mercy, regarded as an affection of passion.
Reply to Objection 2. God acts mercifully, not indeed by going against His justice, but by doing something more than justice; thus a man who pays another two hundred pieces of money, though owing him only one hundred, does nothing against justice, but acts liberally or mercifully. The case is the same with one who pardons an offence committed against him, for in remitting it he may be said to bestow a gift. Hence the Apostle calls remission a forgiving: "Forgive one another, as Christ has forgiven you" (Ephesians 4:32). Hence it is clear that mercy does not destroy justice, but in a sense is the fulness thereof. And thus it is said: "Mercy exalteth itself above judgment" (James 2:13).
Source: Summa Theologica (New Advent)