Letter 101, to Cledonius

Letter 101, to Cledonius — Nicene and Post-Nicene Fathers, series 2, vol. 7 (Charles Gordon Browne & James Edward Swallow, 1894) (public domain)

If anyone does not believe that Holy Mary is the Mother of God, he is severed from the Godhead. If anyone should assert that He passed through the Virgin as through a channel, and was not at once divinely and humanly formed in her (divinely, because without the intervention of a man; humanly, because in accordance with the laws of gestation), he is in like manner godless. If any assert that the Manhood was formed and afterward was clothed with the Godhead, he too is to be condemned. For this were not a Generation of God, but a shirking of generation. If any introduce the notion of Two Sons, one of God the Father, the other of the Mother, and discredits the Unity and Identity, may he lose his part in the adoption promised to those who believe aright. For God and Man are two natures, as also soul and body are; but there are not two Sons or two Gods. For neither in this life are there two manhoods; though Paul speaks in some such language of the inner and outer man. And (if I am to speak concisely) the Saviour is made of elements which are distinct from one another (for the invisible is not the same with the visible, nor the timeless with that which is subject to time), yet He is not two Persons. God forbid! For both natures are one by the combination, the Deity being made Man, and the Manhood deified or however one should express it. And I say different Elements, because it is the reverse of what is the case in the Trinity; for There we acknowledge different Persons so as not to confound the persons; but not different Elements, for the Three are One and the same in Godhead.

'If anyone does not admit that holy Mary is Mother of God, he is cut off from the Godhead.' Theotokos as a test of belief in the Incarnation, half a century before Ephesus.

The single paragraph beginning with the key sentence (the series of anathemas on the Incarnation).