Objection 1. It would seem that it is not fitting that God should adopt sons. For, as jurists say, no one adopts anyone but a stranger as his son. But no one is a stranger in relation to God, Who is the Creator of all. Therefore it seems unfitting that God should adopt.
Objection 2. Further, adoption seems to have been introduced in default of natural sonship. But in God there is natural sonship, as set down in the I, 27, 2. Therefore it is unfitting that God should adopt.
Objection 3. Further, the purpose of adopting anyone is that he may succeed, as heir, the person who adopts him. But it does not seem possible for anyone to succeed God as heir, for He can never die. Therefore it is unfitting that God should adopt.
On the contrary, It is written (Ephesians 1:5) that "He hath predestinated us unto the adoption of children of God." But the predestination of God is not ineffectual. Therefore God does adopt some as His sons.
I answer that, A man adopts someone as his son forasmuch as out of goodness he admits him as heir to his estate. Now God is infinitely good: for which reason He admits His creatures to a participation of good things; especially rational creatures, who forasmuch as they are made to the image of God, are capable of Divine beatitude. And this consists in the enjoyment of God, by which also God Himself is happy and rich in Himself--that is, in the enjoyment of Himself. Now a man's inheritance is that which makes him rich. Wherefore, inasmuch as God, of His goodness, admits men to the inheritance of beatitude, He is said to adopt them. Moreover Divine exceeds human adoption, forasmuch as God, by bestowing His grace, makes man whom He adopts worthy to receive the heavenly inheritance; whereas man does not make him worthy whom he adopts; but rather in adopting him he chooses one who is already worthy.
Reply to Objection 1. Considered in his nature man is not a stranger in respect to God, as to the natural gifts bestowed on him: but he is as to the gifts of grace and glory; in regard to which he is adopted.
Reply to Objection 2. Man works in order to supply his wants: not so God, Who works in order to communicate to others the abundance of His perfection. Wherefore, as by the work of creation the Divine goodness is communicated to all creatures in a certain likeness, so by the work of adoption the likeness of natural sonship is communicated to men, according to Romans 8:29: "Whom He foreknew... to be made conformable to the image of His Son."
Reply to Objection 3. Spiritual goods can be possessed by many at the same time; not so material goods. Wherefore none can receive a material inheritance except the successor of a deceased person: whereas all receive the spiritual inheritance at the same time in its entirety without detriment to the ever-living Father.
Yet it might be said that God ceases to be, according as He is in us by faith, so as to begin to be in us by vision, as a gloss says on Romans 8:17: "If sons, heirs also."
Source: Summa Theologica (New Advent)