The Truth of His Nature did not seem sufficiently explained by the name of Son, unless, in addition, its peculiar force as proper to Him were expressed, so signifying its distinctness from all beside. For in that, besides Son, he calleth Him also the Only-Begotten, he cut off altogether all suspicion of adoption, the Nature of the Only-Begotten guaranteeing the truth of the name.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Trin. vi. 39