From His Seven Books of Hypotyposes or Outlines

The substance of the Son is not a substance devised extraneously, nor is it one introduced out of nothing; but it was born of the substance of the Father, as the reflection of light or as the steam of water. For the reflection is not the sun itself, and the steam is not the water itself, nor yet again is it anything alien; neither is He Himself the Father, nor is He alien, but He is an emanation from the substance of the Father, this substance of the Father suffering the while no partition. For as the sun remains the same and suffers no diminution from the rays that are poured out by it, so neither did the substance of the Father undergo any change in having the Son as an image of itself.

Theognostus, moreover, himself adds words to this effect: He who has offended against the first term and the second, may be judged to deserve smaller punishment; but he who has also despised the third, can no longer find pardon. For by the first term and the second, he says, is meant the teaching concerning the Father and the Son; but by the third is meant the doctrine committed to us with respect to the perfection and the partaking of the Spirit. And with the view of confirming this, he adduces the word spoken by the Saviour to the disciples: “I have yet many things to say unto you, but ye cannot bear them now. But when the Holy Spirit is come, He will teach you.”