13 Do not imagine that you are to do something beyond a man's ability. The Evangelist John himself did this. He soared beyond the flesh, beyond the earth which he trod, beyond the seas which he looked upon, beyond the air in which the fowls fly, beyond the sun, the moon, the stars, beyond all the spirits unseen, beyond his own mind, by the very reason of his rational soul. Soaring beyond all these, pouring out his soul above him, whither did he arrive? What did he see? “In the beginning was the Word, and the Word was with God.” If, therefore, you see no separation in the light, why do you seek a separation in the work? See God, see His Word inhering to the Word speaking, that the speaker speaks not by syllables, but this his speaking is a shining out in the brightness of wisdom. What is said of the Wisdom itself? “It is the radiance of eternal light.” Observe the radiance of the sun. The sun is in the heaven, and spreads out its brightness over all lands and over all seas, and it is simply a corporal light.
If, indeed, you can separate the brightness from the sun, then separate the Word from the Father. I am speaking of the sun. One small, slender flame of a lamp, which can be extinguished by one breath, spreads its light over all that lies near it: you see the light generated by the flame spread out; you see its emission, but not a separation. Understand, then, beloved brethren, that the Father, and the Son, and the Holy Ghost are inseparably united in themselves; that this Trinity is one God; that all the works of the one God are the works of the Father, of the Son, and of the Holy Ghost. All the rest which follows, and which refers to the discourse of our Lord Jesus Christ, now that a discourse is due to you tomorrow also, be present that you may hear.
Source: Tractates on the Gospel of John (New Advent)