Despite what many Orthodox theologians say who have been influenced by Karl Barth, Christ did not assume a fallen human nature, but received the entirety of humanity in a pre-fallen state as a new Adam restoring fallen man to Paradise. This is the biblical and patristic tradition expressed in the Orthodox Church. Below are some quotes to help illuminate this teaching further.
Fr. George Florovsky taught:
It must be stressed that in the Incarnation the Word assumes the original human nature, innocent and free from original sin, without any stain. This does not violate the fullness of nature, nor does this affect the Savior’s likeness to us sinful people. For sin does not belong to human nature, but is a parasitic and abnormal growth. This point was vigorously stressed by St. Gregory of Nyssa and particularly by St. Maximus the Confessor in connection with their teaching of the will as the seat of sin. In the Incarnation the Word assumes the first-formed human nature, created "in the image of God," and thereby the image of God is again re-established in man.
St. Gregory Palamas taught:
As the evil one procured our twofold death by means of his single spiritual death, so the good Lord healed this twofold death of ours through His single bodily death, and through the one Resurrection of His Body gave us a twofold resurrection. By means of His bodily death He destroyed him who had power over our souls and bodies in death, and rescued us from his tyranny over them both. The evil one clothed himself in the serpent to deceive man, but the Word of God put on man's nature to trick the trickster. He received this nature in its undeceived and pure state, and kept it so to the end, offering it as first fruits to the Father for sanctification from ourselves for ourselves.
St. Justin Martyr taught:
According to the will of God, Jesus Christ the Son of God has been born without sin, of a virgin sprung from the stock of Abraham.
St. Athanasius of Alexandria taught:
We have, then, now stated in part, as far as it was possible, and as ourselves had been able to understand, the reason of His bodily appearing; that it was in the power of none other to turn the corruptible to incorruption, except the Saviour Himself, that had at the beginning also made all things out of nought and that none other could create anew the likeness of God's image for men, save the Image of the Father; and that none other could render the mortal immortal, save our Lord Jesus Christ, Who is the Very Life; and that none other could teach men of the Father, and destroy the worship of idols, save the Word, that orders all things and is alone the true Only-begotten Son of the Father. But since it was necessary also that the debt owing from all should be paid again: for, as I have already said, it was owing that all should die, for which special cause, indeed, He came among us: to this intent, after the proofs of His Godhead from His works, He next offered up His sacrifice also on behalf of all, yielding His Temple to death in the stead of all, in order firstly to make men quit and free of their old trespass, and further to show Himself more powerful even than death, displaying His own body incorruptible, as first-fruits of the resurrection of all. And do not be surprised if we frequently repeat the same words on the same subject. For since we are speaking of the counsel of God, therefore we expound the same sense in more than one form, lest we should seem to be leaving anything out, and incur the charge of inadequate treatment: for it is better to submit to the blame of repetition than to leave out anything that ought to be set down. The body, then, as sharing the same nature with all, for it was a human body, though by an unparalleled miracle it was formed of a virgin only, yet being mortal, was to die also, conformably to its peers. But by virtue of the union of the Word with it, it was no longer subject to corruption according to its own nature, but by reason of the Word that had come to dwell in it it was placed out of the reach of corruption. And so it was that two marvels came to pass at once, that the death of all was accomplished in the Lord's body, and that death and corruption were wholly done away by reason of the Word that was united with it. For there was need of death, and death must needs be suffered on behalf of all, that the debt owing from all might be paid. Whence, as I said before, the Word, since it was not possible for Him to die, as He was immortal, took to Himself a body such as could die, that He might offer it as His own in the stead of all, and as suffering, through His union with it, on behalf of all, Bring to nought Him that had the power of death, that is the devil; and might deliver them who through fear of death were all their lifetime subject to bondage.
St. John the Damascene taught:
So then, after the assent of the holy Virgin, the Holy Spirit descended on her, according to the word of the Lord which the angel spoke, purifying her, and granting her power to receive the divinity of the Word, and likewise power to bring forth. And then was she overshadowed by the enhypostatic Wisdom and Power of the most high God, the Son of God Who is of like essence with the Father as of Divine seed, and from her holy and most pure blood He formed flesh animated with the spirit of reason and thought, the first-fruits of our compound nature: not by procreation but by creation through the Holy Spirit: not developing the fashion of the body by gradual additions but perfecting it at once, He Himself, the very Word of God, standing to the flesh in the relation of subsistence. For the divine Word was not made one with flesh that had an independent pre-existence, but taking up His abode in the womb of the holy Virgin, He unreservedly in His own subsistence took upon Himself through the pure blood of the eternal Virgin a body of flesh animated with the spirit of reason and thought, thus assuming to Himself the first-fruits of man's compound nature, Himself, the Word, having become a subsistence in the flesh. So that He is at once flesh, and at the same time flesh of God the Word, and likewise flesh animated, possessing both reason and thought. Wherefore we speak not of man as having become God, but of God as having become Man. For being by nature perfect God, He naturally became likewise perfect Man: and did not change His nature nor make the dispensation an empty show, but became, without confusion or change or division, one in subsistence with the flesh, which was conceived of the holy Virgin, and animated with reason and thought, and had found existence in Him, while He did not change the nature of His divinity into the essence of flesh, nor the essence of flesh into the nature of His divinity, and did not make one compound nature out of His divine nature and the human nature He had assumed.
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Thus, therefore, we confess that the nature of the Godhead is wholly and perfectly in each of its subsistences, wholly in the Father, wholly in the Son, and wholly in the Holy Spirit. Wherefore also the Father is perfect God, the Son is perfect God, and the Holy Spirit is perfect God. In like manner, too, in the Incarnation of the Trinity of the One God the Word of the Holy Trinity, we hold that in one of its subsistences the nature of the Godhead is wholly and perfectly united with the whole nature of humanity, and not part united to part. The divine Apostle in truth says that in Him dwelleth all the fulness of the Godhead bodily, that is to say in His flesh. And His divinely-inspired disciple, Dionysius, who had so deep a knowledge of things divine, said that the Godhead as a whole had fellowship with us in one of its own subsistences. But we shall not be driven to hold that all the subsistences of the Holy Godhead, to wit the three, are made one in subsistence with all the subsistences of humanity. For in no other respect did the Father and the Holy Spirit take part in the incarnation of God the Word than according to good will and pleasure. But we hold that to the whole of human nature the whole essence of the Godhead was united. For God the Word omitted none of the things which He implanted in our nature when He formed us in the beginning, but took them all upon Himself, body and soul both intelligent and rational, and all their properties. For the creature that is devoid of one of these is not man. But He in His fulness took upon Himself me in my fulness, and was united whole to whole that He might in His grace bestow salvation on the whole man. For what has not been taken cannot be healed.