16 For God so loved the world, as to give his only begotten Son; that whosoever believeth in him, may not perish, but may have life everlasting.
Chrysostom: Having said, Even so must the Son of man be lifted up [John 3:14], alluding to His death; lest His hearer should be cast down by His words, forming some human notion of Him, and thinking of His death as an evil, He corrects this by saying, that He who was given up to death was the Son of God, and that His death would be the source of life eternal. . . . As if He said, Marvel not that I must be lifted up, that you may be saved: for so it seemeth good to the Father, who hath so loved you, that He hath given His Son to suffer for ungrateful and careless servants. The text, God so loved the world, shews intensity of love. For great indeed and infinite is the distance between the two. He who is without end, or beginning of existence, Infinite Greatness, loved those who were of earth and ashes, creatures laden with sins innumerable. And the act which springs from the love is equally indicative of its vastness. For God gave not a servant, or an Angel, or an Archangel, but His Son. Again, had He had many sons, and given one, this would have been a very great gift; but now He has given His Only Begotten Son.
Hilary: If it were only a creature given up for the sake of a creature, such a poor and insignificant loss were no great evidence of love. They must be precious things which prove our love, great things must evidence its greatness. God, in love to the world, gave His Son, not an adopted Son, but His own, even His Only Begotten. Here is proper Sonship, birth, truth: no creation, no adoption, no lie: here is the test of love and charity, that God sent His own and only begotten Son to save the world.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. IV (London: Rivington, 1845).