Summa Theologica, by St. Thomas Aquinas, , at sacred-texts.com
We must now come to consider whether adoption befits Christ: and under this head there are four points of inquiry:
(1) Whether it is fitting that God should adopt sons?
(2) Whether this is fitting to God the Father alone?
(3) Whether it is proper to man to be adopted to the sonship of God?
(4) Whether Christ can be called the adopted Son?
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 FP, Q, A. 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 (Eph. 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 Rom. 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 Rom. 8:17: "If sons, heirs also."
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.
Objection 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 (Jn. 20:17): "I ascend to My Father and to your Father." Therefore Christ's Father alone has adopted sons.
Objection 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 FP, Q, A, OBJ; cf. FP, Q, A. 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 Jn. 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 (Jn. 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.
Reply to Objection 1: All human individuals are not of one individual nature, so that there need be one operation and one effect of them all, as is the case in God. Consequently in this respect no comparison is possible.
Reply to Objection 2: By adoption we are made the brethren of Christ, as having with Him the same Father: Who, nevertheless, is His Father in one way, and ours in another. Whence pointedly our Lord says, separately, "My Father," and "Your Father" (Jn. 20:17). For He is Christ's Father by natural generation; and this is proper to Him: whereas He is our Father by a voluntary operation, which is common to Him and to the Son and Holy Ghost: so that Christ is not the Son of the whole Trinity, as we are.
Reply to Objection 3: As stated above (A, ad 2), adoptive sonship is a certain likeness of the eternal Sonship: just as all that takes place in time is a certain likeness of what has been from eternity. Now man is likened to the splendor of the Eternal Son by reason of the light of grace which is attributed to the Holy Ghost. Therefore adoption, though common to the whole Trinity, is appropriated to the Father as its author; to the Son, as its exemplar; to the Holy Ghost, as imprinting on us the likeness of this exemplar.
Objection 1: It would seem that it is not proper to the rational nature to be adopted. For God is not said to be the Father of the rational creature, save by adoption. But God is called the Father even of the irrational creature, according to Job 38:28: "Who is father of the rain? Or who begot the drops of dew?" Therefore it is not proper to the rational creature to be adopted.
Objection 2: Further, by reason of adoption some are called sons of God. But to be sons of God seems to be properly attributed by the Scriptures to the angels; according to Job 1:6: "On a certain day when the sons of God came to stand before the Lord." Therefore it is not proper to the rational creature to be adopted.
Objection 3: Further, whatever is proper to a nature, belongs to all that have that nature: just as risibility belongs to all men. But to be adopted does not belong to every rational nature. Therefore it is not proper to human nature.
On the contrary, Adopted sons are the "heirs of God," as is stated Rom. 8:17. But such an inheritance belongs to none but the rational nature. Therefore it is proper to the rational nature to be adopted.
I answer that, As stated above (A, ad 3), the sonship of adoption is a certain likeness of natural sonship. Now the Son of God proceeds naturally from the Father as the Intellectual Word, in oneness of nature with the Father. To this Word, therefore, something may be likened in three ways. First, on the part of the form but not on the part of its intelligibility: thus the form of a house already built is like the mental word of the builder in its specific form, but not in intelligibility, because the material form of a house is not intelligible, as it was in the mind of the builder. In this way every creature is like the Eternal Word; since it was made through the Word. Secondly, the creature is likened to the Word, not only as to its form, but also as to its intelligibility: thus the knowledge which is begotten in the disciple's mind is likened to the word in the mind of the master. In this way the rational creature, even in its nature, is likened to the Word of God. Thirdly, a creature is likened to the Eternal Word, as to the oneness of the Word with the Father, which is by reason of grace and charity: wherefore our Lord prays (John 17:21, 22): "That they may be one in Us . . . as We also are one." And this likeness perfects the adoption: for to those who are thus like Him the eternal inheritance is due. It is therefore clear that to be adopted belongs to the rational creature alone: not indeed to all, but only to those who have charity; which is "poured forth in our hearts by the Holy Ghost" (Rom. 5:5); for which reason (Rom. 8:15) the Holy Ghost is called "the Spirit of adoption of sons."
Reply to Objection 1: God is called the Father of the irrational creature, not properly speaking, by reason of adoption, but by reason of creation; according to the first-mentioned participation of likeness.
Reply to Objection 2: Angels are called sons of God by adoptive sonship, not that it belongs to them first; but because they were the first to receive the adoption of sons.
Reply to Objection 3: Adoption is a property resulting not from nature, but from grace, of which the rational nature is capable. Therefore it need not belong to every rational nature: but every rational creature must needs be capable of adoption.
Objection 1: It would seem that Christ as man is the adopted Son of God. For Hilary says (De Trin. ii) speaking of Christ: "The dignity of power is not forfeited when carnal humanity [*Some editions read 'humilitas'---'the humility or lowliness of the flesh'] is adopted." Therefore Christ as man is the adopted Son of God.
Objection 2: Further, Augustine says (De Praedest. Sanct. xv) that "by the same grace that Man is Christ, as from the birth of faith every man is a Christian." But other men are Christians by the grace of adoption. Therefore this Man is Christ by adoption: and consequently He would seem to be an adopted son.
Objection 3: Further, Christ, as man, is a servant. But it is of greater dignity to be an adopted son than to be a servant. Therefore much more is Christ, as man, an adopted Son.
On the contrary, Ambrose says (De Incarn. viii): "We do not call an adopted son a natural son: the natural son is a true son." But Christ is the true and natural Son of God, according to 1 Jn. 5:20: "That we may . . . be in His true Son, Jesus Christ." Therefore Christ, as Man, is not an adopted Son.
I answer that, Sonship belongs properly to the hypostasis or person, not to the nature; whence in the FP, Q, A we have stated that Filiation is a personal property. Now in Christ there is no other than the uncreated person or hypostasis, to Whom it belongs by nature to be the Son. But it has been said above (A, ad 2), that the sonship of adoption is a participated likeness of natural sonship: nor can a thing be said to participate in what it has essentially. Therefore Christ, Who is the natural Son of God, can nowise be called an adopted Son.
But according to those who suppose two persons or two hypostases or two supposita in Christ, no reason prevents Christ being called the adopted Son of God.
Reply to Objection 1: As sonship does not properly belong to the nature, so neither does adoption. Consequently, when it is said that "carnal humanity is adopted," the expression is metaphorical: and adoption is used to signify the union of human nature to the Person of the Son.
Reply to Objection 2: This comparison of Augustine is to be referred to the principle because, to wit, just as it is granted to any man without meriting it to be a Christian, so did it happen that this man without meriting it was Christ. But there is a difference on the part of the term: because by the grace of union Christ is the natural Son; whereas another man by habitual grace is an adopted son. Yet habitual grace in Christ does not make one who was not a son to be an adopted son, but is a certain effect of Filiation in the soul of Christ, according to Jn. 1:14: "We saw His glory . . . as it were of the Only-begotten of the Father; full of grace and truth."
Reply to Objection 3: To be a creature, as also to be subservient or subject to God, regards not only the person, but also the nature: but this cannot be said of sonship. Wherefore the comparison does not hold.