Council of Ephesus — as excerpted in the Catena Aurea on John 1:1

“(Gest. Conc. Eph.) Wherefore in one place divine Scripture calls Him the Son, in another the Word, in another the Brightness of the Father; names severally meant to guard against blasphemy. For, forasmuch as thy son is of the same nature with thyself, the Scripture wishing to shew that the Substance of the Father and the Son is one, sets forth the Son of the Father, born of the Father, the Only-Begotten. Next, since the terms birth and son, convey the idea of passibleness, therefore it calls the Son the Word, declaring by that name the impassibility of His Nativity. But inasmuch as a father with us is necessarily older than his son, lest thou shouldest think that this applied to the Divine nature as well, it calls the Only-Begotten the Brightness of the Father; for brightness, though arising from the sun, is not posterior to it. Understand then that Brightness, as revealing the coeternity of the Son with the Father; Word as proving the impassibility of His birth, and Son as conveying His consubstantiality.”