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." _______________________
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 sons.
Obj. 3: Further, it is written (Gal. 4:4, 5, 6): "God sent His Son . . . that we might receive the adoption of sons. And because you are sons of God, God hath sent the Spirit of His Son into your hearts, crying: 'Abba' (Father)." Therefore it belongs to Him to adopt, Who has the Son and the Holy Ghost. But this belongs to the Father alone. Therefore it befits the Father alone to adopt.
On the contrary, It belongs to Him to adopt us as sons, Whom we can call Father; whence it is written (Rom. 8:15): "You have received the spirit of adoption of sons, whereby we cry: 'Abba' (Father)." But when we say to God, "Our Father," we address the whole Trinity: as is the case with the other names which are said of God in respect of creatures, as stated in the First Part (Q. 33, A. 3, Obj. 1; cf. Q. 45, A. 6). Therefore to adopt is befitting to the whole Trinity.
I answer that, There is this difference between an adopted son of God and the natural Son of God, that the latter is "begotten not made"; whereas the former is made, according to John 1:12: "He gave them power to be made the sons of God." Yet sometimes the adopted son is said to be begotten, by reason of the spiritual regeneration which is by grace, not by nature; wherefore it is written (James 1:18): "Of His own will hath He begotten us by the word of truth." Now although, in God, to beget belongs to the Person of the Father, yet to produce any effect in creatures is common to the whole Trinity, by reason of the oneness of their Nature: since, where there is one nature, there must needs be one power and one operation: whence our Lord says (John 5:19): "What things soever the Father doth, these the Son also doth in like manner." Therefore it belongs to the whole Trinity to adopt men as sons of God.