It is well for us that in another passage (the apostle) calls Christ “the image of the invisible God.” For will it not follow with equal force from that passage, that Christ is not truly God, because the apostle places Him in the image of God, if, (as Marcion contends,) He is not truly man because of His having taken on Him the form or image of a man? For in both cases the true substance will have to be excluded, if image (or “fashion”) and likeness and form shall be claimed for a phantom. But since he is truly God, as the Son of the Father, in His fashion and image, He has been already by the force of this conclusion determined to be truly man, as the Son of man, “found in the fashion” and image “of a man.” For when he propounded Him as thus “found” in the manner of a man, he in fact affirmed Him to be most certainly human. For what is found, manifestly possesses existence. Therefore, as He was found to be God by His mighty power, so was He found to be man by reason of His flesh, because the apostle could not have pronounced Him to have “become obedient unto death,” if He had not been constituted of a mortal substance.
Tertullian
The Five Books Against Marcion, Book V
Chapter XX