The Commonitory of Vincent of Lérins, For the Antiquity and Universality of the Catholic Faith Against the Profane Novelties of All Heresies 14

Jesus Christ Man in Truth, not in Semblance.

[38.] But when we use the word “Person,” and say that God became man by means of a Person, there is reason to fear that our meaning may be taken to be, that God the Word assumed our nature merely in imitation, and performed the actions of man, being man not in reality, but only in semblance, just as in a theatre, one man within a brief space represents several persons, not one of whom himself is. For when one undertakes to sustain the part of another, he performs the offices, or does the acts, of the person whose part he sustains, but he is not himself that person. So, to take an illustration from secular life and one in high favour with the Manichees, when a tragedian represents a priest or a king, he is not really a priest or a king. For, as soon as the play is over, the person or character whom he represented ceases to be. God forbid that we should have anything to do with such nefarious and wicked mockery. Be it the infatuation of the Manichees, those preachers of hallucination, who say that the Son of God, God, was not a human person really and truly, but that He counterfeited the person of a man in feigned conversation and manner of life.

[39.] But the Catholic Faith teaches that the Word of God became man in such wise, that He took upon Him our nature, not feignedly and in semblance, but in reality and truth, and performed human actions, not as though He were imitating the actions of another, but as performing His own, and as being in reality the person whose part He sustained. Just as we ourselves also, when we speak, reason, live, subsist, do not imitate men, but are men. Peter and John, for instance, were men, not by imitation, but by being men in reality. Paul did not counterfeit an apostle, or feign himself to be Paul, but was an apostle, was Paul. So, also, that which God the Word did, in His condescension, in assuming and having flesh, in speaking, acting, and suffering, through the instrumentality of flesh, yet without any marring of His own divine nature, came in one word to this:—He did not imitate or feign Himself to be perfect man, but He shewed Himself to be very man in reality and truth. Therefore, as the soul united to the flesh, but yet not changed into flesh, does not imitate man, but is man, and man not feignedly but substantially, so also God the Word, without any conversion of Himself, in uniting Himself to man, became man, not by confusion, not by imitation, but by actually being and subsisting. Away then, once and for all, with the notion of His Person as of an assumed fictitious character, where always what is is one thing, what is counterfeited another, where the man who acts never is the man whose part he acts. God forbid that we should believe God the Word to have taken upon Himself the person of a man in this illusory way. Rather let us acknowledge that while His own unchangeable substance remained, and while He took upon Himself the nature of perfect man, Himself actually was flesh, Himself actually was man, Himself actually was personally man; not feignedly, but in truth, not in imitation, but in substance; not, finally, so as to cease to be when the performance was over, but so as to be, and continue to be substantially and permanently.