Gregory the Great
Epistula 28 ad Flavianum
chapters 3-4

Suscepta est a maiestate humilitas, a virtute infirmitas, ab aeternitate mortalitas; et ad resolvendum condicionis nostrae debitum, natura inviolabilis naturae est unita passibili, ut, quod nostris remediis congruebat, unus atque idem mediator Dei et hominum, homo Iesus Christus, et mori posset ex uno, et mori non posset ex altero. Lowliness is assured by majesty, weakness by power, mortality by eternity. To pay the debt of our sinful state, a nature that is incapable of suffering was joined to one that could suffer. Thus, in keeping with the healing that we needed, one and the same mediator between God and men, the man Jesus Christ, was able to die in one nature, and unable to die in the other.
In integra ergo veri hominis perfectaque natura verus natus est Deus, totus in suis, totus in nostris. Nostra autem dicimus, quae in nobis ab initio Creator condidit, et quae reparanda suscepit. He who is true God was therefore born in the complete and perfect nature of a true man, whole in his own nature, whole in ours. By our nature we nean what the Creator has fashioned in us from the beginning, and took to himself in order to restore it.
Nam illa quae deceptor intulit, et homo deceptus admisit, nullum habuerunt in Salvatore vestigium. Nec, quia communionem humanarum subiit infirmitatum, ideo nostrorum fuit particeps delictorum. For in the Savior there was no trace of what the deceiver introduced and man, being misled, allowed to enter. It does not follow that because he submitted to sharing in hour human weakness he therefore shared in our sins.
Assumpsit formam servi sine sorde peccati, humana augens, divina non minuens: quia exinanitio illa, qua se invisibilis visibilem praebuit, et Creator ac Dominus omnium rerum unus voluit esse mortalium, inclinatio fuit miserationis, non defectio potestatis. Proinde, qui manens in forma Dei fecit hominem, idem in forma servi factus est homo. He took the nature of a servant without the stain of sin, enlarging our humanity without diminishing his divinity. He emptied himself; though invisible he made himself visible, though Creator and Lord of all things he chose to be one of us mortal men.
Ingreditur ergo haec mundi infima Filius Dei, de caelesti sede descendens, et a paterna gloria non recedens, novo ordine, nova nativitate generatus. Yet this was the condescension of compassion, not the loss of omnipotence. So he who is in the nature of God had created man, became in the nature of a servant, man himself.
Novo ordine, quia, invisibilis in suis, visibilis factus est in nostris; incomprehensibilis, voluit comprehendi; ante tempora manens, esse coepit ex tempore; universitatis Dominus servilem formam, obumbrata maiestatis suae immensitate, suscepit; impassibilis Deus non dedignatus est homo esse passibilis, et immortalis mortis legibus subiacere. He was born in a new condition, for, invisible in his own nature, he became visible in ours. Beyond our grasp, he chose to come witin our grasp. Existing before time began, he began to exist at a moment in time. Lord of the universe, he hid his infinite glory and took the nature of a servant. Incapable of suffering as God, he did not refuse to be a man, capable of sufering. Immortal, he chose to be subject to the laws of death.
Qui enim verus est Deus, idem verus est homo, et nullum est in hac unitate mendacium, dum invicem sunt et humilitas hominis et altitudo Deitatis. He who is true God is also true man. There is no falsehood in this unity as long as the lowliness of man and the preeminence of God coexist in mutual relationship.
Sicut enim Deus non mutatur miseratione, ita homo non consumitur dignitate. Agit enim utraque forma, cum alterius communione, quod proprium est: Verbo scilicet operante quod Verbi est, et carne exsequente quod carnis est. As God does not change by his condescension, so man is not swallowed up by being exalted. Each nature exercises its own activity, in communion with the other. The word does what is proper to the Word, the flesh fulfills what is proper to the flesh.
Unum horum coruscat miraculis, aliud succumbit iniuriis. Et sicut Verbum ab aequalitate paternae gloriae non recedit, ita caro naturam nostri generis non relinquit. One nature is respendent with miracles, ths other falls victim to injuries. As the Word does not lose equality with the Father's glory, so the flesh does not leave behind the nature of our race.
Unus enim idemque est, quod saepe dicendum est, vere Dei Filius, et vere hominis filius. Deus per id quod in principio erat Verbum, et Verbum erat apud Deum, et Deus erat Verbum; homo per id quod Verbum caro factum est, et habitavit in nobis. One and the same person—this must be said over and over again—is truly the Son of God and truly the Son of Man. He is God in virtue of the fact that in the beginning was the Word, and the Word was with God, and the Word was God. He is man in virtue of the fact that the Word was made flesh, and dwelt among us.