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 John 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 I, 32, 3 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 (1, 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 John 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.
Source: Summa Theologica (New Advent)