John Cassian, On the Incarnation of the Lord, Against Nestorius VI.15: He shows that those who patronize this false teaching acknowledge two Christs

But still, I had begun to say, that as you certainly make out two Christs this very matter must be illustrated and made clear. Tell me, I pray you, you who sever Christ from the Son of God, how can you confess in the Creed that Christ was begotten of God? For you say: “I believe in God the Father, and in Jesus Christ His Son.” Here then you have Jesus Christ the Son of God: but you say that it was not the same Son of God who was born of Mary. Therefore there is one Christ of God, and another of Mary. In your view then there are two Christs. For, though in the Creed you do not deny Christ, you say that the Christ of Mary is another than the one whom you confess in the Creed. But perhaps you say that Christ was not begotten of God: how then do you say in the Creed: “I believe in Jesus Christ the Son of God?” You must then either deny the Creed or confess that Christ is the Son of God. But if you confess in the Creed that Christ is the Son of God, you must also confess that the same Christ, the Son of God, is of Mary. Or if you make out another Christ of Mary, you certainly make the blasphemous assertion that there are two Christs.