In the Creed, what is meant by the expression “Light of Light?”
Under the figure of light, the Fathers illustrated the incomprehensible generation of the Son of God from the Father.
When we look at the sun, we see light; from that light is generated the light seen everywhere below; yet the one and the other are one light, indivisible and of one nature. In like manner, God the Father is everlasting Light (1Jo 1:5; see Q431), dwelling in unapproachable light (1Ti 6:16; see Q167, Q367).
Another image of the Trinity offered by the Fathers is the sun, which radiates both light and warmth with his Word and his Spirit. All such images are aids to reverence, not explanations of the mystery.
From the Father is begotten the Son of God, who is likewise everlasting Light; and God and his Word are one in essence and undivided, as St Athanasius explains:
There never was a time when God was not the Father… The Son, being the brightness of the eternal Light, is certainly himself eternal; for as the Light exists always, it is clear that its Brightness exists always. It is by its shining that we perceive that light exists, and there cannot be light that does not illumine. Since, therefore, the Father is eternal, the Son, being Light of Light, is eternal. (St Dionysius of Alexandria, quoted by St Athanasius, Refutation and Defense; St Athanasius, On the Opinion of Dionysius, 15)
The Church sings this confession every evening: “O Gladsome Light of the holy glory of the immortal Father, heavenly, holy, blessed Jesus Christ!” (Vespers, Phos Hilaron).
