Now this word “I am” expresses His eternal subsistence. For if He is the reflection of the eternal light, He must also be eternal Himself. For if the light subsists for ever, it is evident that the reflection also subsists for ever. And that this light subsists, is known only by its shining; neither can there be a light that does not give light. We come back, therefore, to our illustrations. If there is day, there is light; and if there is no such thing, the sun certainly cannot be present. If, therefore, the sun had been eternal, there would also have been endless day. Now, however, as it is not so, the day begins when the sun rises, and it ends when the sun sets. But God is eternal light, having neither beginning nor end. And along with Him there is the reflection, also without beginning, and everlasting. The Father, then, being eternal, the Son is also eternal, being light of light; and if God is the light, Christ is the reflection; and if God is also a Spirit, as it is written, “God is a Spirit,” Christ, again, is called analogously Spirit.
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 6 (Roberts and Donaldson, 1886), public domain (Public domain).