40 I defer the consideration of the words, which is in the bosom of the Father, to a more appropriate place. My present enquiry is into the sense of Only-begotten, and the claim upon us which that sense may make. And first let us see whether the word mean, as you assert, a perfect creature of God; Only-begotten being equivalent to perfect, and Son a synonym for creature. But John described the Only-begotten Son as God, not as a perfect creature.
His words, Which is in the bosom of the Father, show that he anticipated these blasphemous designations; and, indeed, he had heard his Lord say, For God so loved the world that He gave His Only-begotten Son, that whosoever believes in Him should not perish but have everlasting life. God, Who loved the world, gave His Only-begotten Son as a manifest token of His love. If the evidence of His love be this, that He bestowed a creature upon creatures, gave a worldly being on the world's behalf, granted one raised up from nothing for the redemption of objects equally raised up from nothing, this cheap and petty sacrifice is a poor assurance of His favour towards us.
Gifts of price are the evidence of affection: the greatness of the surrender of the greatness of the love. God, Who loved the world, gave not an adopted Son, but His own, His Only-begotten. Here is personal interest, true Sonship, sincerity; not creation, or adoption, or pretence. Herein is the proof of His love and affection, that He gave His own, His Only-begotten Son.
Source: On the Trinity (New Advent)