If, therefore, Holy Scripture, providing for our need, ever putting before us what is intangible, clothes it in flesh, does it not make an image of what is thus invested with our nature, and brought to the level of our desires, yet invisible? A certain conception through the senses thus takes place in the brain, which was not there before, and is transmitted to the judicial faculty, and added to the mental store. Gregory, who is so eloquent about God, says that the mind, which is set upon getting beyond corporeal things, is incapable of doing it. For the invisible things of God since the creation of the world are made visible through images. (Rom. 1.20) We see images in creation which remind us faintly of God, as when, for instance, we speak of the holy and adorable Trinity, imaged by the sun, or light, or burning rays, or by a running fountain, or a full river, or by the mind, speech, or the spirit within us, or by a rose tree, or a sprouting flower, or a sweet fragrance.
St. John Damascene
Apology Against Those Who Decry Holy Images
Part I