That although from the beginning He had been the Son of God, yet He had to be begotten again according to the flesh
In the second Psalm: “The Lord said unto me, You are my Son; this day have I begotten You. Ask of me, and I will give You the nations for Your inheritance, and the bounds of the earth for Your possession.” Also in the Gospel according to Luke: “And it came to pass, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and she was filled with the Holy Ghost, and she cried out with a loud voice, and said, Blessed are you among women, and blessed is the fruit of your womb. And whence does this happen to me, that the mother of my Lord should come to me?” Also Paul to the Galatians: “But when the fullness of the time had come, God sent His Son, horn of a woman.” Also in the Epistle of John: “Every spirit which confesses that Jesus Christ has come in the flesh is of God. But whosoever denies that He has come in the flesh is not of God, but is of the spirit of Antichrist.”
Source: The Treatises of Cyprian (New Advent)