7 Understand the nature of the Word of God, by whom all things were made, to be such that you cannot think of any part of the Word as passing, and, from being future, becoming past. He remains as He is, and He is everywhere in His entirety. He comes when He is manifested, and departs when He is concealed. But whether concealed or manifested, He is present with us as light is present to the eyes both of the seeing and of the blind; but it is felt to be present by the man who sees, and absent by him who is blind. In like manner, the sound of the voice is near alike to the hearing and to the deaf, but it makes its presence known to the former and is hidden from the latter. But what is more wonderful than what happens in connection with the sound of our voices and our words, a thing, for-truth, which passes away in a moment? For when we speak, there is no place for even the next syllable till after the preceding one has ceased to sound; nevertheless, if one hearer be present, he hears the whole of what we say, and if two hearers be present, both hear the same, and to each of them it is the whole; and if a multitude listen in silence, they do not break up the sounds like loaves of bread, to be distributed among them individually, but all that is uttered is imparted to all and to each in its entirety. Consider this, and say if it is not more incredible that the abiding word of God should not accomplish in the universe what the passing word of man accomplishes in the ears of listeners, namely, that as the word of man is present in its entirety to each and all of the hearers, so the Word of God should be present in the entirety of His being at the same moment everywhere.
8. There is, therefore, no reason to fear in regard to the small body of the Lord in His infancy, lest in it the Godhead should seem to have been straitened. For it is not in vast size but in power that God is great: He has in His providence given to ants and to bees senses superior to those given to asses and camels; He forms the huge proportions of the fig-tree from one of the minutest seeds, although many smaller plants spring from much larger seeds; He also has furnished the small pupil of the eye with the power which, by one glance, sweeps over almost the half of heaven in a moment; He diffuses the whole fivefold system of the nerves over the body from one centre and point in the brain; He dispenses vital motion throughout the whole body from the heart, a member comparatively small; and by these and other similar things, He, who in small things is great, mysteriously produces that which is great from things which are exceedingly little. Such is the greatness of His power that He is conscious of no difficulty in that which is difficult. It was this same power which originated, not from without, but from within, the conception of a child in the Virgin's womb: this same power associated with Himself a human soul, and through it also a human body— in short, the whole human nature to be elevated by its union with Him— without His being thereby lowered in any degree; justly assuming from it the name of humanity, while amply giving to it the name of Godhead. The body of the infant Jesus was brought forth from the womb of His mother, still a virgin, by the same power which afterwards introduced His body when He was a man through the closed door into the upper chamber. Here, if the reason of the event is sought out, it will no longer be a miracle; if an example of a precisely similar event is demanded, it will no longer be unique. Let us grant that God can do something which we must admit to be beyond our comprehension. In such wonders the whole explanation of the work is the power of Him by whom it is wrought.
Source: Letters (New Advent)