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.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: vi. de Trin. c. 40