“Tell me then, before the Lord Jesus Christ was born of His mother Mary, had God a Son or had He not? You cannot deny that He had, for never yet was there either a son without a father, or a father without a son: because as a son is so called with reference to a father, so is a father so named with reference to a son.” 

John Cassian (c. 360-485), The Seven Books of John Cassian on the Incarnation of our Lord, Against Nestorius, NPNF, p. 574.