18 The Son, wishing to assure us of the truth of this, His Divine birth, has appointed His works to serve as an illustration, that from the ineffable power displayed in ineffable deeds we may learn the lesson of the ineffable birth. For instance, when water was made wine, and five loaves satisfied five thousand men, beside women and children, and twelve baskets were filled with the fragments, we see a fact though we cannot understand it; a deed is done though it baffles our reason; the process cannot be followed, though the result is obvious.
It is folly to intrude in the spirit of carping, when the matter into which we enquire is such that we cannot probe it to the bottom. For even as the Father is ineffable because He is Unbegotten, so is the Son ineffable because He is the Only-begotten, since the Begotten is the Image of the Unbegotten. Now it is by the use of our senses and of language that we have to form our conception of an image; and it must be by the same means that we form our idea of that which the image represents.
But in this case we, whose faculties can deal only with visible and tangible things, are straining after the invisible, and striving to grasp the impalpable. Yet we take no shame to ourselves, we reproach ourselves with no irreverence, when we doubt and criticise the mysteries and powers of God. How is He the Son? Whence is He? What did the Father lose by His birth? Of what portion of the Father was He born? So we ask; yet all the while there has been confronting us the evidence of works done to assure us that God's action is not limited by our power of comprehending His methods.
Source: On the Trinity (New Advent)