6 But, beyond reason inflated [with your own wisdom], you presumptuously maintain that you are acquainted with the unspeakable mysteries of God; while even the Lord, the very Son of God, allowed that the Father alone knows the very day and hour of judgment, when He plainly declares, “But of that day and that hour knows no man, neither the Son, but the Father only.” If, then, the Son was not ashamed to ascribe the knowledge of that day to the Father only, but declared what was true regarding the matter, neither let us be ashamed to reserve for God those greater questions which may occur to us.
For no man is superior to his master. If any one, therefore, says to us, “How then was the Son produced by the Father?” we reply to him, that no man understands that production, or generation, or calling, or revelation, or by whatever name one may describe His generation, which is in fact altogether indescribable. Neither Valentinus, nor Marcion, nor Saturninus, nor Basilides, nor angels, nor archangels, nor principalities, nor powers [possess this knowledge], but the Father only who begot, and the Son who was begotten.
Since therefore His generation is unspeakable, those who strive to set forth generations and productions cannot be in their right mind, inasmuch as they undertake to describe things which are indescribable. For that a word is uttered at the bidding of thought and mind, all men indeed well understand. Those, therefore, who have excogitated [the theory of] emissions have not discovered anything great, or revealed any abstruse mystery, when they have simply transferred what all understand to the only-begotten Word of God; and while they style Him unspeakable and unnameable, they nevertheless set forth the production and formation of His first generation, as if they themselves had assisted at His birth, thus assimilating Him to the word of mankind formed by emissions.
Source: Adversus haereses (New Advent)