IV. (3) Their account of the epithet Only Begotten refuted.
Again the third head is concerned with these same folk's impious assertion that the Son of God is called “only-begotten” for this reason that He alone was born of a virgin. To be sure they would not have dared to say this, had they not drunk the poison of Paul of Samosata and Photinus: who said that our Lord Jesus Christ did not exist till He was born of the virgin Mary. But if they wish something else to be understood by their tenet, and do not date Christ's beginning from His mother's womb, they must necessarily assert that there is not one Son of God, but others also were begotten of the most High Father, of whom this one is born of a woman, and therefore called only-begotten, because no other of God's sons underwent this condition of being born. Therefore, wherever they betake themselves, they fall into an abyss of great impiety, if they either maintain that Christ the Lord took His beginning from His mother, or do not believe Him to be the only-begotten of God the Father: since He who was God was born of a mother, and no one was born of the Father except the Word.
Source: Letters (New Advent)