Second Council of Constantinople, Capitulum IV

If anyone shall say that the union of the Word of God to man was only according to grace or energy, or dignity, or equality of honour, or authority, or relation, or effect, or power, or according to good pleasure in this sense that God the Word was pleased with a man, that is to say, that he loved him for his own sake, as says the senseless Theodorus, or [if anyone pretends that this union exists only] so far as likeness of name is concerned, as the Nestorians understand, who call also the Word of God Jesus and Christ, and even accord to the man the names of Christ and of Son, speaking thus clearly of two persons, and only designating disingenuously one Person and one Christ when the reference is to his honour, or his dignity, or his worship; if anyone shall not acknowledge as the Holy Fathers teach, that the union of God the Word is made with the flesh animated by a reasonable and living soul, and that such union is made synthetically and hypostatically, and that therefore there is only one Person, to wit:  our Lord Jesus Christ, one of the Holy Trinity:  let him be anathema.  As a matter of fact the word “union” (τῆς ἑνώςεως) has many meanings, and the partisans of Apollinaris and Eutyches have affirmed that these natures are confounded inter se, and have asserted a union produced by the mixture of both.  On the other hand the followers of Theodorus and of Nestorius rejoicing in the division of the natures, have taught only a relative union.  Meanwhile the Holy Church of God, condemning equally the impiety of both sorts of heresies, recognises the union of God the Word with the flesh synthetically, that is to say, hypostatically.  For in the mystery of Christ the synthetical union not only preserves unconfusedly the natures which are united, but also allows no separation.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.