Reply Obj. 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 Obj. 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 Obj. 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. _______________________
FOURTH ARTICLE [III, Q. 23, Art. 4]
Whether Christ As Man Is the Adopted Son of God?
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.
Obj. 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.
Obj. 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 the First Part (Q. 32, A. 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 (A. 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.