We have been taught that He proceeds forth from God, and in that procession He is generated; so that He is the Son of God, and is called God from unity of substance with God. For God, too, is a Spirit. Even when the ray is shot from the sun, it is still part of the parent mass; the sun will still be in the ray, because it is a ray of the sun—there is no division of substance, but merely an extension. Thus Christ is Spirit of Spirit, and God of God, as light of light is kindled. The material matrix remains entire and unimpaired, though you derive from it any number of shoots possessed of its qualities; so, too, that which has come forth out of God is at once God and the Son of God, and the two are one. In this way also, as He is Spirit of Spirit and God of God, He is made a second in manner of existence—in position, not in nature; and He did not withdraw from the original source, but went forth. This ray of God, then, as it was always foretold in ancient times, descending into a certain virgin, and made flesh in her womb, is in His birth God and man united.
Tertullian
Apology
Chapter XXI