2 This is He Who was begotten from the Godhead according to His nature, and from manhood not after His nature, and from baptism not after His custom; that we might be begotten from manhood according to our nature, and from Godhead not after our nature, and by the Spirit not after our custom. He then was begotten from the Godhead, He that came to a second birth; in order to bring us to the birth that is discoursed of, even His generation from the Father:— not that it should be searched out, but that it should be believed—and His birth from the woman, not that it should be despised, but that it should be exalted.
Now His death on the cross witnesses to His birth from the woman. For He that died was also born. And the Annunciation of Gabriel declares His generation by the Father, namely [the power of the Highest shall overshadow you]. If then it was the power of the Highest, it is plain that it was not the seed of mortal man. So then His conception in the womb is bound up with His death on the cross; and His first generation is bound up with the declaration of the Angel; in order that whoso denies His birth may be confuted by His crucifixion, and whoso supposes that His beginning was from Mary, may be admonished that His Godhead is before all; so that whoever has concluded His beginning to be corporeal, [may be proved to err hereby that His issuing forth from the Father is narrated].
The Father begot Him, and through Him created the creatures. Flesh bare Him and through Him slew lusts. Baptism brought him forth, that through Him it might wash away stains. Sheol brought Him forth, that through Him its treasures might be emptied out. He came to us from beside His Father by the way of them that are born: and by the way of them that die, He went forth to go to His Father; so that by His coming through birth, His advent might be seen; and by His returning through resurrection, His departure might be confirmed.
Source: On Our Lord (New Advent)