Why, it is asked, does He speak in the plural number, We speak that we do know? Because the speaker being the Only-Begotten Son of God, He would shew that the Father was in the Son, and the Son in the Father, and the Holy Ghost from both, proceeding indivisibly.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom. in Oct. Pent.