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 Obj. 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 Obj. 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 Rom. 8:29: "Whom He foreknew . . . to be made
conformable to the image of His Son."

Reply Obj. 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
Rom. 8:17: "If sons, heirs also."
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SECOND ARTICLE [III, Q. 23, Art. 2]

Whether It Is Fitting That the Whole Trinity Should Adopt?

Objection 1: It would seem unfitting that the whole Trinity should
adopt. For adoption is said of God in likeness to human custom. But
among men those only adopt who can beget: and in God this can be
applied only to the Father. Therefore in God the Father alone can
adopt.

Obj. 2: Further, by adoption men become the brethren of Christ,
according to Rom. 8:29: "That He might be the first-born among many
brethren." Now brethren are the sons of the same father; wherefore
our Lord says (John 20:17): "I ascend to My Father and to your
Father." Therefore Christ's Father alone has adopted so