Chapter II.
On the incarnation with respect to the union of natures.
Concerning the incarnate Word itself, three things present themselves for our consideration, namely the union of natures, the fullness of charisms, and the endurance of sufferings for the redemption of the human race.
Concerning the union of natures, these three things are to be considered for understanding the mystery of the incarnation, namely the work, the mode, and the time.
Concerning the work of the incarnation, these things are to be held according to the Christian faith: that the incarnation is an operation of the Trinity, through which the assumption of flesh by the Godhead and the union of the Godhead with flesh takes place; such that the assumption is not only of sensible flesh, but also of a rational spirit according to the power of vegetating, sensing, and understanding; such also that the union does not take place in a unity of nature, but of person, not a human person, but a divine one, not an assumed person, but of the one assuming; not the person of just any, but the person of the Word alone: in which person so great a union takes place that whatever is said of the Son of God is said of the son of man, and conversely, excepting however those cases in which the union is expressed, or in which a negation is enclosed.
Now the reason for understanding the aforesaid is this: because the work of the incarnation is not only from the first principle insofar as it is effective in producing, but also insofar as it is restorative by remedying, satisfying, and reconciling. Since therefore the incarnation, insofar as it denotes some effect, is from the first principle, which does all things by reason of its supreme power; and substance, power, and operation are united and in every way undivided in the three persons: hence it is that the operation of the incarnation must necessarily proceed from the whole Trinity.
But because it is from the first principle insofar as it is restorative by remedying; and the whole human race had fallen and been corrupted, not only with respect to the soul, but also the flesh: hence it was necessary that the whole be assumed, so that the whole might be healed. - And since the part of the flesh is more known to us and more distant from God, so that the naming might be more expressive, and the greater humiliation might be expressed, and the deeper condescension might be unfolded: hence it is that this work is called not "inanimation" but incarnation.
Again, because it is from the first principle, insofar as it is restorative by making satisfaction, and satisfaction is not made except by one who both ought and is able: and none ought except man, nor is any able except God: it was necessary that in the satisfaction there be a concurrence of both natures together, namely the divine and the human. And because it is impossible that the divine nature should concur with another as a part toward the constitution of a third; nor that it should pass into another nature, nor that another nature should pass into it, on account of its most perfect simplicity and immutability: hence it is that the Godhead and humanity are not united in a unity of nature nor of accident, and therefore they are united in a unity of person and hypostasis. And since the divine nature can subsist in no supposit other than in its own hypostasis, therefore that union cannot be in the hypostasis or person of man, but of God: and through this, by that union the first principle in one of its hypostases made itself the supposit of human nature: and thus there is there only one personality and personal unity, namely on the part of the one assuming.
Finally, because it is from the first principle, insofar as it is restorative by reconciling: and the one who reconciles is a mediator, and mediation properly befits the Son of God: therefore the incarnation also. For it belongs to the Mediator to be a middle between man and God for leading man back to divine knowledge, to divine conformity, and to divine filiation. But none is more fitting to be the middle than the person who produces and is produced, who is the middle of the three persons: and none is more fitting to lead man back to divine knowledge than the Word, by whom the Father declares himself, who is unitable to flesh, just as a word is to a voice; nor is any more fitting to lead back to divine conformity than him who is the Image of the Father; none is more fitting to lead back to adoptive filiation than the natural Son: and through this, none is more fitting to become the son of man than the Son of God himself.
Since therefore the Son of man and of God is entirely the same by reason of the incarnation, and "whatever things are the same as one and the same thing are the same as each other": hence it is that the communication of idioms necessarily occurs, unless there is a term in which some repugnance is included: such as those in which is included a relation of the union of one nature to the other, such as to unite, to be incarnated, to assume, and to be assumed; or the negation of something whose opposite belongs to the other, such as to begin to exist, to be created, and similar things, in which an objection is brought against the aforementioned rule for the aforesaid reason.