Christ's eternity being proved from the Apostle's teaching, St. Ambrose admonishes us that the Divine Generation is not to be thought of after the fashion of human procreation, nor to be too curiously pried into. With the difficulties thence arising he refuses to deal, saying that whatsoever terms, taken from our knowledge of body, are used in speaking of this Divine Generation, must be understood with a spiritual meaning.
62. Hear now another argument, showing clearly the eternity of the Son. The Apostle says that God's Power and Godhead are eternal, and that Christ is the Power of God— for it is written that Christ is “the Power of God and the Wisdom of God.” If, then, Christ is the Power of God, it follows that, forasmuch as God's Power is eternal, Christ also is eternal.
63. You can not, then, heretic, build up a false doctrine from the custom of human procreation, nor yet gather the wherewithal for such work from our discourse, for we cannot compass the greatness of infinite Godhead, “of Whose greatness there is no end,” in our straitened speech. If you should seek to give an account of a man's birth, you must needs point to a time. But the Divine Generation is above all things; it reaches far and wide, it rises high above all thought and feeling. For it is written: “No man comes to the Father, save by Me.” Whatsoever, therefore, thou dost conceive concerning the Father— yea, be it even His eternity— you can not conceive anything concerning Him save by the Son's aid, nor can any understanding ascend to the Father save through the Son. “This is My dearly-beloved Son,” the Father says. “Is” mark you— He Who is, what He is, forever. Hence also David is moved to say: “O Lord, Your Word abides for ever in heaven,” — for what abides fails neither in existence nor in eternity.
64. Do you ask me how He is a Son, if He have not a Father existing before Him? I ask of you, in turn, when, or how, do you think that the Son was begotten. For me the knowledge of the mystery of His generation is more than I can attain to, — the mind fails, the voice is dumb— ay, and not mine alone, but the angels' also. It is above Powers, above Angels, above Cherubim, Seraphim, and all that has feeling and thought, for it is written: “The peace of Christ, which passes all understanding.” If the peace of Christ passes all understanding, how can so wondrous a generation but be above all understanding?
65. Do thou, then (like the angels), cover your face with your hands, for it is not given you to look into surpassing mysteries! We are suffered to know that the Son is begotten, not to dispute upon the manner of His begetting. I cannot deny the one; the other I fear to search into, for if Paul says that the words which he heard when caught up into the third heaven might not be uttered, how can we explain the secret of this generation from and of the Father, which we can neither hear nor attain to with our understanding?
66. But if you will constrain me to the rule of human generation, that you may be allowed to say that the Father existed before the Son, then consider whether instances, taken from the generation of earthly creatures, are suitable to show forth the Divine Generation. If we speak according to what is customary among men, you cannot deny that, in man, the changes in the father's existence happen before those in the son's. The father is the first to grow, to enter old age, to grieve, to weep. If, then, the son is after him in time, he is older in experience than the son. If the child comes to be born, the parent escapes not the shame of begetting.
67. Why take such delight in that rack of questioning? You hear the name of the Son of God; abolish it, then, or acknowledge His true nature. You hear speak of the womb— acknowledge the truth of undoubted begetting. Of His heart— know that here is God's word. Of His right hand— confess His power. Of His face— acknowledge His wisdom. These words are not to be understood, when we speak of God, as when we speak of bodies. The generation of the Son is incomprehensible, the Father begets impassibly, and yet of Himself and in ages inconceivably remote has very God begotten very God. The Father loves the Son, and you anxiously examine His Person; the Father is well pleased in Him, you, joining the Jews, look upon Him with an evil eye; the Father knows the Son, and you join the heathen in reviling Him.
Source: On the Christian Faith (De fide) (New Advent)