Thomas Aquinas

“ Hence in the fifth Council (Constantinople II, coll. viii, can. 5) we read: "Since the unity may be understood in many ways, those who follow the impiety of Apollinaris and Eutyches, professing the destruction of what came together" (i.e. destroying both natures) , "confess a union by mingling; but the followers of Theodore and Nestorius, maintaining division, introduce a union of purpose. But the Holy Church of God, rejecting the impiety of both these treasons, confesses a union of the Word of God with flesh, by composition, which is in subsistence." ”

“ And hence we must say that the union of which we are speaking is not really in God, except only in our way of thinking; but in the human nature, which is a creature, it is really. Therefore we must say it is something created. ”

“ If anyone after the uniting divides the subsistences in the one Christ, only joining them in a union of dignity or authority or power, and not rather in a concourse of natural union, let him be anathema. ”

“ Personality pertains of necessity to the dignity of a thing, and to its perfection so far as it pertains to the dignity and perfection of that thing to exist by itself (which is understood by the word "person") . ”

“ Therefore the union took place in the person.
I answer that, Person has a different meaning from "nature." For nature, as has been said [3861] (A [1] ) , designates the specific essence which is signified by the definition. And if nothing was found to be added to what belongs to the notion of the species, there would be no need to distinguish the nature from the suppositum of the nature (which is the individual subsisting in this nature) , because every individual subsisting in a nature would be altogether one with its nature. ”

“ For some conceded one person in Christ, but maintained two hypostases, or two supposita, saying that a man, composed of body and soul, was from the beginning of his conception assumed by the Word of God. ”

“ Whatever accrues after the completion of the being comes accidentally, unless it be taken into communion with the complete being, just as in the resurrection the body comes to the soul which pre-exists, yet not accidentally, because it is assumed unto the same being, so that the body has vital being through the soul ”

“ Those who came into this world before Christ's coming, whatsoever eminency of righteousness they may have had, could not, on being divested of the body, at once be admitted into the bosom of the heavenly country, seeing that He had not as yet come Who, by His own descending, should place the souls of the righteous in their everlasting seat. ”

“ Thirdly, because to the hypostasis alone are attributed the operations and the natural properties, and whatever belongs to the nature in the concrete; for we say that this man reasons, and is risible, and is a rational animal. So likewise this man is said to be a suppositum, because he underlies [supponitur] whatever belongs to man and receives its predication. Therefore, if there is any hypostasis in Christ besides the hypostasis of the Word, it follows that whatever pertains to man is verified of some other than the Word, e.g. ”

“ And therefore he did not concede that the man was really the Son of God, but His instrument. Hence Cyril says (Epist. ad Monach. Aegyptii) : "The Scripture does not affirm that this Emmanuel," i.e. Christ, "was assumed for the office of an instrument, but as God truly humanized," i.e. made man. ”