11 The Father then begot Him not in such wise as any man could understand, but as Himself only knows. For we profess not to tell in what manner He begot Him, but we insist that it was not in this manner. And not we only are ignorant of the generation of the Son from the Father, but so is every created nature. Speak to the earth, if perchance it may teach you: and though thou inquire of all things which are upon the earth, they shall not be able to tell you.
For the earth cannot tell the substance of Him who is its own potter and fashioner. Nor is the earth alone ignorant, but the sun also: for the sun was created on the fourth day, without knowing what had been made in the three days before him; and he who knows not the things made in the three days before him, cannot tell forth the Maker Himself. Heaven will not declare this: for at the Father's bidding the heaven also was like smoke established by Christ. Nor shall the heaven of heavens declare this, nor the waters which are above the heavens. Why then art you cast down, O man, at being ignorant of that which even the heavens know not?
Nay, not only are the heavens ignorant of this generation, but also every angelic nature. For if any one should ascend, were it possible, into the first heaven, and perceiving the ranks of the Angels there should approach and ask them how God begot His own Son, they would say perhaps, “We have above us beings greater and higher; ask them.” Go up to the second heaven and the third; attain, if you can, to Thrones, and Dominions, and Principalities, and Powers: and even if any one should reach them, which is impossible, they also would decline the explanation, for they know it not.
Source: Catechetical Lectures (New Advent)