The cause of all our good is the Lord and divine love. For to love is, properly speaking, to will good to someone. Therefore, since the will of God is the cause of things, good comes to us because God loves us. And God’s love is the cause of the good of nature: “You love everything which exists” (Wis 11:2 5). It is also the cause of the good which is grace: “I have loved you with an. everlasting love, and so I have drawn you” i.e., through grace (Jer 3 1:3). But it is because of his great love that he gives us the good of glory. So he shows us here, from four standpoints, that this love of God is the greatest.
First, from the person of the one loving, because it is God who loves, and immeasurably. So he says, For God so loved: “He has loved the people; all the holy ones are in his hand” (Dt 33:3). Secondly, from the condition of the one who is loved, because it is man, a bodily creature of the world, i.e., existing in sin: “God shows his love for us, because while we were still his enemies, we were reconciled to God by the death of his Son” (Rom 5:8). Thus he says, the world. Thirdly, from the greatness of his gifts, for love is shown by a gift; as Gregory says: “The proof of love is given by action.” But God has given us the greatest of gifts, his Only Begotten Son, and so he says, that he gave his Only Begotten Son. “God did not spare his own Son, but delivered him up for all of us” (Rom 8:32).
Commentary on the Gospel of John, ch. 3, l. 3, sect. 477