CONFESS then the same truth in respect of the actual nativity of the Lord, as in respect of everything else. Believe that God was born when He would, for you do not deny that He could do what He would; unless possibly you think that that power which belonged to Him for all other things was deficient as regards Himself, and that His Omnipotence though proceeding from Him and penetrating all things, was insufficient to bring about His own nativity. In the case of the Lord's nativity you bring this as an objection against me: No one gives birth to one who is anterior in time: and in regard of the birth which Almighty God underwent you say that the one who is born ought to be of one substance with the one who bears; as if you had to do with human laws as in the case of any ordinary man, to whom you might bring the impossibility as an objection, as you include him in the weakness of earthly things. You say that for all men there are common conditions of birth, and but one law of generation; and that a thing could not possibly happen to one man only out of the whole of humanity, which God has forbidden to happen to all. You do not understand of whom you are speaking; nor do you see of whom you are talking; for He is the Author of all conditions, and the very Law of all natures, through whom exists whatever man can do, and whatever man cannot do: for He certainly has laid down the limits of both; viz., how far his powers should extend, and the bounds beyond which his weakness should not advance. How wildly then do you bring human impossibilities as an objection in the case of Him, who possesses all powers and possibilities. If you estimate the Person of the Lord by earthly weaknesses, and measure God's Omnipotence by human rules, you will most certainly fail to find anything which seems appropriate to God as concerns the sufferings of His Body. For if it can seem to you unreasonable that Mary could give birth to God who was anterior to her, how will it seem reasonable that God was crucified by men? And yet the same God who was crucified Himself predicted: "Shall a man afflict God, for you afflict Me?" If then we cannot think that the Lord was born of a Virgin because He who was born was anterior to her who bore Him, how can we believe that God had blood? And yet it was said to the Ephesian elders: "Feed the Church of God which He has purchased with His own Blood." Finally how can we think that the Author of life was Himself deprived of life: And yet Peter says: "Ye have killed the Author of life." No one who is set on earth can be in heaven: and how does the Lord Himself say: "The Son of man who is in heaven"? If then you think that God was not born of a Virgin because the one who is born must be of one substance with the one who bears, how will you believe that different things can be produced from different natures? Thus according to you the wind did not suddenly bring the quails, nor did the manna fall, nor was water turned into wine nor were many thousands of men fed with a few loaves, nor did the blind man receive his sight after the clay had been put on him. But if all these things seem incredible and contrary to nature, unless we believe that they were wrought by God, why should you deny in the matter of His nativity, what you admit in the matter of His works? Or was He unable to contribute to His own nativity and advent what He did not refuse for the succour and profit of men?
THE SEVEN BOOKS OF JOHN CASSIAN ON THE INCARNATION OF THE LORD -- AGAINST NESTORIUS, BOOK VII, CHAPTER IV.