45 But now let us learn what is this faith concerning the Son of God, which the Apostle holds. For though there is no single discourse, among the many which he delivered concerning the Church's doctrine, in which he mentions the Father without also making confession of the Son, yet, in order to display the truth of the relation which that name conveys with the utmost definiteness of which human language is capable, he speaks thus:— What then? If God be for us, who can be against us?
Who spared not His own Son, but delivered Him up for us. Can Son, by any remaining possibility, be a title received through adoption, when He is expressly called God's own Son? For the Apostle, wishing to make manifest the love of God towards us, uses a kind of comparison, to enable us to estimate how great that love is, when He says that it was His own Son Whom God did not spare. He suggests the thought that this was no sacrifice of an adopted Son, on behalf of those whom He purposed to adopt, of a creature for creatures, but of His Son for strangers, His own Son for those to whom He had willed to give a share in the name of sons.
Seek out the full import of the term, that you may understand the extent of the love. Consider the meaning of own; mark the genuineness of the Sonship which it implies. For the Apostle now describes Him as God's own Son; previously he had often spoken of Him as God's Son, or Son of God. And though many manuscripts, through a want of apprehension on the part of the translators, read in this passage His Son, instead of His own Son, yet the original Greek, the tongue in which the Apostle wrote, is more exactly rendered by His own than by His. And though the casual reader may discern no great difference between His own and His, yet the Apostle, who in all his other statements had spoken of His Son, which is, in the Greek, τὸν ἑαυτοῦ υἱ& 231·ν, in this passage uses the words ὅς γε τοῦ ἰδίου υἱοῦ οὐκ ἐφείσατο, that is, Who spared not His own Son, expressly and emphatically indicating His true Divine nature.
Previously he had declared that through the Spirit of adoption there are many sons; now his object is to point to God's own Son, God the Only-begotten.
Source: On the Trinity (New Advent)