Objection 1. It seems that to be good does not belong to God. For goodness consists in mode, species and order. But these do not seem to belong to God; since God is immense and is not ordered to anything else. Therefore to be good does not belong to God.
Objection 2. Further, the good is what all things desire. But all things do not desire God, because all things do not know Him; and nothing is desired unless it is known. Therefore to be good does not belong to God.
On the contrary, It is written (Lamentations 3:25): "The Lord is good to them that hope in Him, to the soul that seeketh Him."
I answer that, To be good belongs pre-eminently to God. For a thing is good according to its desirableness. Now everything seeks after its own perfection; and the perfection and form of an effect consist in a certain likeness to the agent, since every agent makes its like; and hence the agent itself is desirable and has the nature of good. For the very thing which is desirable in it is the participation of its likeness. Therefore, since God is the first effective cause of all things, it is manifest that the aspect of good and of desirableness belong to Him; and hence Dionysius (Div. Nom. iv) attributes good to God as to the first efficient cause, saying that, God is called good "as by Whom all things subsist."
Reply to Objection 1. To have mode, species and order belongs to the essence of caused good; but good is in God as in its cause, and hence it belongs to Him to impose mode, species and order on others; wherefore these three things are in God as in their cause.
Reply to Objection 2. All things, by desiring their own perfection, desire God Himself, inasmuch as the perfections of all things are so many similitudes of the divine being; as appears from what is said above (Question 4, Article 3). And so of those things which desire God, some know Him as He is Himself, and this is proper to the rational creature; others know some participation of His goodness, and this belongs also to sensible knowledge; others have a natural desire without knowledge, as being directed to their ends by a higher intelligence.
Source: Summa Theologica (New Advent)