Question: "Is the idea of a "fallen nature" a later Western concept that was foreign to the early Fathers? If I remember correctly, I've seen prominent Orthodox theologians use the term "fallen nature". But also I think I've seen other prominent theologians say that the term is a later Western concept that was foreign to the Fathers, and that its usage is incorrect. What is your opinion on this topic?"
I think you are actually referring to the objection that some Orthodox writers have expressed to the phrase "original sin". I would hope that no one has been foolish enough to argue that human nature is not fallen. The services of the Church are full of references to our fallen nature. For example, in the service for the feast of the Ascension alone, there are several such references:
"Not being separated from the bosom of the Father, O most sweet Jesus, and having lived on earth as a man, Thou wast taken up in glory today from the Mount of Olives. And having raised our fallen nature by Thy compassion, Thou didst seat it together with the Father. Wherefore, the heavenly orders of the bodiless were amazed at the wonder and stood in awe and astonishment. They were seized with trembling and magnified Thy love for mankind. With them we on earth also glorify Thy condescension toward us, and Thine Ascension from us, entreating and saying: O Thou Who by Thine Ascension didst fill with infinite joy Thy disciples and the Theotokos who bare Thee, by their prayers deem us also worthy of the joy of Thy chosen ones, for Thy great mercy's sake" (Doxasticon at Lord, I have cried...).
"God is gone up in jubilation, the Lord with the voice of the trumpet, to raise the fallen image of Adam, and to send the Comforting Spirit to sanctify our souls" (Doxasticon at the Vespers Aposticha).
"Thou didst raise up human nature which was fallen into corruption, O Christ, and in Thine Ascension Thou didst exalt us and glorify us together with Thyself" (4th troparion of Ode III of the Canon).
"The majesty of Him Who became poor in the flesh hath been manifestly taken up above the heavens; and our fallen nature hath been honored by sitting with the Father. Let us all make feast, and with one accord let us cry out with jubilation and clap our hands rejoicing" (8th troparion of the Ode IX of the Canon).
There is a tendency among some contemporary Orthodox writers to dismiss anything that they see as being western, and often in the process they end of dismissing important aspects of Orthodox Tradition as well. And in this case, there have been a number of Orthodox writers that have rejected the phrase "original sin." For example:
Original sin: Orthodox doctrine or heresy?, Archimandrite Vassilios Papavassiliou
Ancestral Versus Original Sin: An Overview with Implications for Psychotherapy
by V. Rev. Antony Hughes, M.Div- See more at: http://www.stmaryorthodoxchurch.org/orthodoxy/articles/ancestral_versus_original_sin#sthash.L261SiD4.dpuf
The problem with this approach is that the phrase "original sin" was used and affirmed by the Council of Carthage, in 418 A.D., and this council was officially affirmed by the 6th and 7th Ecumenical Councils. See: Original Sin and Orthodoxy: Reflections on Carthage, and Original Sin and Ephesus: Carthage’s Influence on the East, by Nathaniel McCallum.
Fr. Michael Pomazansky lays out the Orthodox understanding of original sin in his "Orthodox Dogmatic Theology":
"By original sin is meant the sin of Adam, which was transmitted to his descendants and weighs upon them. The doctrine of original sin has great significance in the Christian world-view, because upon it rests a whole series of other dogmas.
The word of God teaches us that through Adam "all have sinned": "By one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned" (Rom. 5:12). "For who will be clean of defilement? No one, if he have lived even a single day upon earth" (Job 14:4-5, Septuagint). "For behold, I was conceived in iniquities, and in sins did my mother bear me" (Ps. 50:5); "the seed of corruption is in me" (Evening Prayers).
The common faith of the ancient Christian Church in the existence of original sin may be seen in the Church's ancient custom of baptizing infants. The Local Council of Carthage in 252, composed of 66 bishops under the presidency of St. Cyprian, decreed the following against heretics: "Not to forbid (the baptism) of an infant who, scarcely born, has sinned in nothing apart from that which proceeds from the flesh of Adam. He has received the contagion of the ancient death through his very birth, and he comes, therefore, the more easily to the reception of the remission of sins in that it is not his own but the sins of another that are remitted." (The same thing is stated in Canon 110 of the "African Code," approved by 217 bishops at Carthage in 419 and ratified by the Council in Trullo (692) and the Seventh Ecumenical Council (787). Canon 110 ends: "On account of this rule of faith even infants, who could have committed as yet no sin themselves, therefore are truly baptized for the remission of sins, in order that what in them is the result of generation may be cleansed by regeneration" (The Seven Ecumenical Councils, Eerdmans ed., p. 497).)
This is the way in which the "Encyclical of the Eastern Patriarchs" defines the result of the fall into sin: "Fallen through the transgression, man became like the irrational creatures. That is, he became darkened and was deprived of perfection and dispassion. But he was not deprived of the nature and power which he had received from the All-good God. For had he been so deprived, he would have become irrational, and thus not a man. But he preserved that nature with which he had been created, and the free, living and active natural power, so that, according to nature, he might choose and do the good, and flee and turn away from evil" ("Encyclical of the Eastern Patriarchs," paragraph 14).
In the history of the ancient Christian Church, Pelagius and his followers denied the inheritance of sin (the heresy of Pelagianism). Pelagius affirmed that every man only repeats the sin of Adam, performing anew his own personal fall into sin, and following the example of Adam because of his own weak will. However, his nature remains the same as when it was created, innocent and pure, the same as that of the first-created Adam. Moreover, disease and death are characteristic of this nature from the creation, and are not the consequences of original sin.
Blessed Augustine stepped out against Pelagius with great power and proof. He cited (a) testimonies from Divine Revelation concerning original sin, (b) the teaching of the ancient shepherds of the Church, (c) the ancient custom of baptizing infants, and (d) the sufferings and misfortunes of men, including infants, which are a consequence of the universal and inherited sinfulness of men. However, Augustine did not escape the opposite extreme, setting forth the idea that in fallen man any independent freedom to do good has been completely annihilated, unless grace comes to his aid.
Out of this dispute in the West there subsequently were formed two tendencies, one of which was followed by Roman Catholicism, and the other by Protestantism. Roman Catholic theologians consider that the consequence of the fall was the removal from men of a supernatural gift of God's grace, after which man remained in his "natural" condition, his nature not harmed but only brought into disorder because flesh, the bodily side, has come to dominate over the spiritual side. Original sin, in this view, consists of the fact that the guilt before God of Adam and Eve has passed to all men.
The other tendency in the West sees in original sin the complete perversion of human nature and its corruption to its very depths, to its very foundations (the view accepted by Luther and Calvin). As for the newer sects of Protestantism, reacting in their turn against the extremes of Luther, they have gone as far as the complete denial of original, inherited sin.
Among the shepherds of the Eastern Church there have been no doubts concerning either the teaching of the inherited ancestral sin in general, or the consequences of this sin for fallen human nature in particular.
Orthodox theology does not accept the extreme points of Blessed Augustine's teaching; but equally foreign to it is the (later) Roman Catholic point of view, which has a very legalistic, formal character. The foundation of the Roman Catholic teaching lies in (a) an understanding of the sin of Adam as an infinitely great offense against God; (b) after this offense there followed the wrath of God; (c) the wrath of God was expressed in the removal of the supernatural gifts of God's grace; and (d) the removal of grace drew after itself the submission of the spiritual principle to the fleshly principle, and a falling deeper into sin and death. From this comes a particular view of the redemption performed by the Son of God: In order to restore the order which had been violated, it was necessary first of all to give satisfaction for the offense given to God, and by this means to remove the guilt of mankind and the punishment that weighs upon him.
The consequences of ancestral sin are accepted by Orthodox theology differently.
After his first fall, man himself departed in soul from God and became unreceptive to the grace of God which was opened to him; he ceased to listen to the divine voice addressed to him, and this led to the further deepening of sin in him.
However, God has never deprived mankind of His mercy, help, grace, and especially His chosen people; and from this people there came forth great righteous men such as Moses, Elijah, Elisha, and the later prophets. The Apostle Paul, in the eleventh chapter of the Epistle to the Hebrews, lists a whole choir of Old Testament righteous ones, saying that they are those "of whom the world was not worthy" (Heb. 11:38). All of them were perfected not without a gift from above, not without the grace of God. The book of Acts cites the words of the first martyr, Stephen, where he says of David that he "found favor (grace) before God, and desired to find a tabernacle of the God of Jacob" (Acts 7:46); that is, to build a Temple for Him. The greatest of the prophets, St. John the Forerunner, was "filled with the Holy Spirit even from his mother's womb" (Luke 1:15). But the Old Testament righteous ones could not escape the general lot of fallen mankind after death, remaining in the darkness of hell, until the founding of the Heavenly Church; that is, until the Resurrection and Ascension of Christ. The Lord Jesus Christ destroyed the gates of hell and opened the way into the Kingdom of Heaven.
One must not see the essence of sin — including original sin — only in the dominance of the fleshly over the spiritual, as Roman Catholic theology teaches. Many sinful inclinations, even very serious ones, have to do with qualities of a spiritual order, such as pride, which, according to the words of the Apostle, is the source, together with lust, of the general sinfulness of the world (1 John 2:15-16). Sin is also present in evil spirits who have no flesh at all. In Sacred Scripture the word "flesh" signifies a condition of not being reborn, a condition opposed to being reborn in Christ "That which it born of the flesh is flesh, and that which is born of the Spirit is spirit" (John 3:6). Of course, this is not to deny that a whole series of passions and sinful inclinations originate in bodily nature, which Sacred Scripture also shows (Romans, ch. 7).
Thus, original sin is understood by Orthodox theology as a sinful inclination which has entered into mankind and become its spiritual disease" (Orthodox Dogmatic Theology, pp. 160-164).
For more on this subject, see:
Rags of Morality: Original Sin and Human Nature, Archpriest Alexander Golubov, Ph.D.
Ancestral vs. Original Sin: A False Dichotomy, by Ephrem Hugh Bensusan
Original Sin According to St. Paul, by Fr. John S. Romanides