The Council of Ephesus (431 AD)

If anyone shall after the [hypostatic] union divide the hypostases in the one Christ, joining them by that connexion alone, which happens according to worthiness, or even authority and power, and not rather by a coming together (συνόδῳ), which is made by natural union (ἕνωσιν φυσικὴν): let him be anathema.

If any one says that Christ, who is also Emmanuel, is One, not [merely] in consequence of connection, but [also] in nature, and does not acknowledge the connection (συνάφεια) of the two natures, that of the Logos and of the assumed manhood, in one Son, as still continuing without mingling; let him be anathema.

Theodore [of Mopsuestia, and in this he was followed by Nestorius,] (and here is his fundamental error,) not merely maintained the existence of two natures in Christ, but of two persons, as, he says himself, no subsistence can be thought of as perfect without personality. As however, he did not ignore the fact that the consciousness of the Church rejected such a double personality in Christ, he endeavoured to get rid of the difficulty, and he repeatedly says expressly: “The two natures united together make only one Person, as man and wife are only one flesh.…If we consider the natures in their distinction, we should define the nature of the Logos as perfect and complete, and so also his Person, and again the nature and the person of the man as perfect and complete. If, on the other hand, we have regard to the union (συνάφεια), we say it is one Person.” The very illustration of the union of man and wife shows that Theodore did not suppose a true union of the two natures in Christ, but that his notion was rather that of an external connection of the two. The expression συνάφεια, moreover, which he selected here instead of the term ἕνωσιν, which he elsewhere employs, being derived from συνάπτω [to join together], expresses only an external connection, a fixing together, and is therefore expressly rejected in later times by the doctors of the Church. And again, Theodore designates a merely external connection also in the phrase already quoted, to the effect that “the Logos dwells in the man assumed as in a temple.” As a temple and the statue set up within it are one whole merely in outward appearance, so the Godhead and manhood in Christ appear only from without in their actuality as one Person, while they remain essentially two Persons.