Objection 1. It seems that not in every work of God are mercy and justice. For some works of God are attributed to mercy, as the justification of the ungodly; and others to justice, as the damnation of the wicked. Hence it is said: "Judgment without mercy to him that hath not done mercy" (James 2:13). Therefore not in every work of God do mercy and justice appear.
Objection 2. Further, the Apostle attributes the conversion of the Jews to justice and truth, but that of the Gentiles to mercy (Romans 15). Therefore not in every work of God are justice and mercy.
Objection 3. Further, many just persons are afflicted in this world; which is unjust. Therefore not in every work of God are justice and mercy.
Objection 4. Further, it is the part of justice to pay what is due, but of mercy to relieve misery. Thus both justice and mercy presuppose something in their works: whereas creation presupposes nothing. Therefore in creation neither mercy nor justice is found.
On the contrary, It is said (Psalm 24:10): "All the ways of the Lord are mercy and truth."
I answer that, Mercy and truth are necessarily found in all God's works, if mercy be taken to mean the removal of any kind of defect. Not every defect, however, can properly be called a misery; but only defect in a rational nature whose lot is to be happy; for misery is opposed to happiness. For this necessity there is a reason, because since a debt paid according to the divine justice is one due either to God, or to some creature, neither the one nor the other can be lacking in any work of God: because God can do nothing that is not in accord with His wisdom and goodness; and it is in this sense, as we have said, that anything is due to God. Likewise, whatever is done by Him in created things, is done according to proper order and proportion wherein consists the idea of justice. Thus justice must exist in all God's works. Now the work of divine justice always presupposes the work of mercy; and is founded thereupon. For nothing is due to creatures, except for something pre-existing in them, or foreknown. Again, if this is due to a creature, it must be due on account of something that precedes. And since we cannot go on to infinity, we must come to something that depends only on the goodness of the divine will--which is the ultimate end. We may say, for instance, that to possess hands is due to man on account of his rational soul; and his rational soul is due to him that he may be man; and his being man is on account of the divine goodness. So in every work of God, viewed at its primary source, there appears mercy. In all that follows, the power of mercy remains, and works indeed with even greater force; as the influence of the first cause is more intense than that of second causes. For this reason does God out of abundance of His goodness bestow upon creatures what is due to them more bountifully than is proportionate to their deserts: since less would suffice for preserving the order of justice than what the divine goodness confers; because between creatures and God's goodness there can be no proportion.
Reply to Objection 1. Certain works are attributed to justice, and certain others to mercy, because in some justice appears more forcibly and in others mercy. Even in the damnation of the reprobate mercy is seen, which, though it does not totally remit, yet somewhat alleviates, in punishing short of what is deserved.
In the justification of the ungodly, justice is seen, when God remits sins on account of love, though He Himself has mercifully infused that love. So we read of Magdalen: "Many sins are forgiven her, because she hath loved much" (Luke 7:47).
Reply to Objection 2. God's justice and mercy appear both in the conversion of the Jews and of the Gentiles. But an aspect of justice appears in the conversion of the Jews which is not seen in the conversion of the Gentiles; inasmuch as the Jews were saved on account of the promises made to the fathers.
Reply to Objection 3. Justice and mercy appear in the punishment of the just in this world, since by afflictions lesser faults are cleansed in them, and they are the more raised up from earthly affections to God. As to this Gregory says (Moral. xxvi, 9): "The evils that press on us in this world force us to go to God."
Reply to Objection 4. Although creation presupposes nothing in the universe; yet it does presuppose something in the knowledge of God. In this way too the idea of justice is preserved in creation; by the production of beings in a manner that accords with the divine wisdom and goodness. And the idea of mercy, also, is preserved in the change of creatures from non-existence to existence.
Source: Summa Theologica (New Advent)