Objection 1. It seems that there is not justice in God. For justice is divided against temperance. But temperance does not exist in God: neither therefore does justice.
Objection 2. Further, he who does whatsoever he wills and pleases does not work according to justice. But, as the Apostle says: "God worketh all things according to the counsel of His will" (Ephesians 1:11). Therefore justice cannot be attributed to Him.
Objection 3. Further, the act of justice is to pay what is due. But God is no man's debtor. Therefore justice does not belong to God.
Objection 4. Further, whatever is in God, is His essence. But justice cannot belong to this. For Boethius says (De Hebdom.): "Good regards the essence; justice the act." Therefore justice does not belong to God.
On the contrary, It is said (Psalm 10:8): "The Lord is just, and hath loved justice."
I answer that, There are two kinds of justice. The one consists in mutual giving and receiving, as in buying and selling, and other kinds of intercourse and exchange. This the Philosopher (Ethic. v, 4) calls commutative justice, that directs exchange and intercourse of business. This does not belong to God, since, as the Apostle says: "Who hath first given to Him, and recompense shall be made him?" (Romans 11:35). The other consists in distribution, and is called distributive justice; whereby a ruler or a steward gives to each what his rank deserves. As then the proper order displayed in ruling a family or any kind of multitude evinces justice of this kind in the ruler, so the order of the universe, which is seen both in effects of nature and in effects of will, shows forth the justice of God. Hence Dionysius says (Div. Nom. viii, 4): "We must needs see that God is truly just, in seeing how He gives to all existing things what is proper to the condition of each; and preserves the nature of each in the order and with the powers that properly belong to it."
Reply to Objection 1. Certain of the moral virtues are concerned with the passions, as temperance with concupiscence, fortitude with fear and daring, meekness with anger. Such virtues as these can only metaphorically be attributed to God; since, as stated above (Question 20, Article 1), in God there are no passions; nor a sensitive appetite, which is, as the Philosopher says (Ethic. iii, 10), the subject of those virtues. On the other hand, certain moral virtues are concerned with works of giving and expending; such as justice, liberality, and magnificence; and these reside not in the sensitive faculty, but in the will. Hence, there is nothing to prevent our attributing these virtues to God; although not in civil matters, but in such acts as are not unbecoming to Him. For, as the Philosopher says (Ethic. x, 8), it would be absurd to praise God for His political virtues.
Reply to Objection 2. Since good as perceived by intellect is the object of the will, it is impossible for God to will anything but what His wisdom approves. This is, as it were, His law of justice, in accordance with which His will is right and just. Hence, what He does according to His will He does justly: as we do justly what we do according to law. But whereas law comes to us from some higher power, God is a law unto Himself.
Reply to Objection 3. To each one is due what is his own. Now that which is directed to a man is said to be his own. Thus the master owns the servant, and not conversely, for that is free which is its own cause. In the word debt, therefore, is implied a certain exigence or necessity of the thing to which it is directed. Now a twofold order has to be considered in things: the one, whereby one created thing is directed to another, as the parts of the whole, accident to substance, and all things whatsoever to their end; the other, whereby all created things are ordered to God. Thus in the divine operations debt may be regarded in two ways, as due either to God, or to creatures, and in either way God pays what is due. It is due to God that there should be fulfilled in creatures what His will and wisdom require, and what manifests His goodness. In this respect, God's justice regards what befits Him; inasmuch as He renders to Himself what is due to Himself. It is also due to a created thing that it should possess what is ordered to it; thus it is due to man to have hands, and that other animals should serve him. Thus also God exercises justice, when He gives to each thing what is due to it by its nature and condition. This debt however is derived from the former; since what is due to each thing is due to it as ordered to it according to the divine wisdom. And although God in this way pays each thing its due, yet He Himself is not the debtor, since He is not directed to other things, but rather other things to Him. Justice, therefore, in God is sometimes spoken of as the fitting accompaniment of His goodness; sometimes as the reward of merit. Anselm touches on either view where he says (Prosolog. 10): "When Thou dost punish the wicked, it is just, since it agrees with their deserts; and when Thou dost spare the wicked, it is also just; since it befits Thy goodness."
Reply to Objection 4. Although justice regards act, this does not prevent its being the essence of God; since even that which is of the essence of a thing may be the principle of action. But good does not always regard act; since a thing is called good not merely with respect to act, but also as regards perfection in its essence. For this reason it is said (De Hebdom.) that the good is related to the just, as the general to the special.
Source: Summa Theologica (New Advent)