9 The manner of this birth is therefore a secret confined to the Two. If any one lays upon his personal incapacity his failure to solve the mystery, in spite of the certainty that Father and Son stand to Each Other in those relations, he will be still more pained at the ignorance to which I confess. I, too, am in the dark, yet I ask no questions. I look for comfort to the fact that Archangels share my ignorance, that Angels have not heard the explanation, and worlds do not contain it, that no prophet has espied it and no Apostle sought for it, that the Son Himself has not revealed it.
Let such pitiful complaints cease. Whoever you are that search into these mysteries, I do not bid you resume your exploration of height and breadth and depth; I ask you rather to acquiesce patiently in your ignorance of the mode of Divine generation, seeing that you know not how His creatures come into existence. Answer me this one question:— Do your senses give you any evidence that you yourself were begotten? Can you explain the process by which you became a father? I do not ask whence you drew perception, how you obtained life, whence your reason comes, what is the nature of your senses of smell, touch, sight, hearing; the fact that we have the use of all these is the evidence that they exist.
What I ask is:— How do you give them to your children? How do you ingraft the senses, lighten the eyes, implant the mind? Tell me, if you can. You have, then, powers which you do not understand, you impart gifts which you cannot comprehend. You are calmly indifferent to the mysteries of your own being, profanely impatient of ignorance concerning the mysteries of God's.
Source: On the Trinity (New Advent)