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 Romans 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 (2, ad 3), the sonship of adoption is a certain likeness of natural sonship. Now the Son of God proceeds naturally from the Father as 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" (Romans 5:5); for which reason (Romans 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.
Source: Summa Theologica (New Advent)