One of the most difficult aspects of our Faith is to begin to understand the notion of the Holy Trinity. One of the Fathers I have found most helpful here is St. Gregory Nazianzus (c. 329-389).
Although St. Gregory used illustrations of the sun and its light, and the mind and its working in depicting the relation between the Father and the Son, yet he warned that those illustrations are not identical with the full reality of the Triune God:
“I have very carefully considered this matter in my own mind, and have looked at it in every point of view, in order to find some illustration of this most important subject, but I have been unable to discover anything on earth with which to compare the nature of the Godhead. For even if I did happen upon some tiny likeness it escaped me for the most part, and left me down below with my example.
I picture to myself an eye, a fountain, a river, as others have done before, to see if they first might be analogous to the Father, the second to the Son, and the third to the Holy Ghost. For in these there is no distinction in time, nor are they torn away from their connexion with each other, though they seem to be parted by three personalities. But I was afraid in the first place that I should present a flow in the Godhead, incapable of standing still ; and secondly that by this figure a numerical unity would be introduced. For the eye and the spring and the river are numerically one, though in different forms.
Again I thought of the sun and a ray and light. But here again there was a fear lest people should get an idea of composition in the uncompounded Nature, such as there is in the sun and the things that are in the sun. And the second place lest we should give Essence to the Father but deny Personality to the others, and make Them only Powers of God, existing in Him and not Personal. For neither the ray nor the light is a sun, but they are only effulgences from the sun, and qualities of its essence. And lest we should thus, as far as the illustration goes, attribute both Being and Not-being to God, which is even more monstrous.”
St. Gregory Nazianzus, 5th Theological Oration (on the Holy Spirit), Articles XXXI and XXXII.