Thomas Aquinas

“ 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 ”

“ 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. ”

“ 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. ”