Doctrine of the Immaculate Conception
The Eastern Orthodox Church denies the Roman Catholic doctrine of the Immaculate Conception, as it was proclaimed by a Bull of Pope Pius IX in 1854 (Ieffabilis Deus). The doctrine maintains that at the moment of Her conception, the Virgin Mary was cleansed of ancestral sin. In his encyclical, Pope Pius quotes from the 17th century Pope Alexander VII, "who authoritatively and decisively declared the mind of the Church":
Concerning the most Blessed Virgin Mary, Mother of God, ancient indeed is that devotion of the faithful based on the belief that her soul, in the first instant of its creation and in the first instant of the soul’s infusion into the body, was, by a special grace and privilege of God, in view of the merits of Jesus Christ, her Son and the Redeemer of the human race, preserved free from all stain of original sin. And in this sense have the faithful ever solemnized and celebrated the Feast of the Conception.
Eastern vs. Western understanding of "original" sin and grace
From an Eastern Orthodox perspective, the doctrine of the Immaculate Conception follows logically from the Roman Catholic doctrine of original sin. Orthodox theologian Protopresbyter Michael Pomazanski (1888-1988) explains:
According to the Roman teaching, the burden of the sin of our first ancestors consists in the removal from mankind of a supernatural gift of grace. But here there arose a theological question: if mankind had been deprived of the gifts of grace, then how is one to understand the words of the Archangel addressed to Mary: “Rejoice, thou that art full of Grace, the Lord is with thee. Blessed art thou among women.... Thou hast found Grace with God” (Luke 1: 28, 30)? One could only conclude that the Most Holy Virgin Mary had been removed from the general law of the “deprivation of grace” and of the guilt of the sin of Adam. And since her life was holy from her birth, consequently she received, in the form of an exception, a supernatural gift, a grace of sanctity, even before her birth, that is, at her conception. Such a deduction was made by the Latin theologians. They called this removal a “privilege” of the Mother of God.1
The Eastern Orthodox understanding of "original sin" (generally referred to as "ancestral" rather than "original") is completely different from that of the Roman Catholic Church. The difference lies in how grace is understood within Orthodox theology vis a vis Roman Catholic theology. The Roman Catholic Church understands grace as something that is essentially created, and thereby "dispensable", whereas the Orthodox understanding that grace is something uncreated. An old edition of the Catholic Encyclopedia (1911 ed.) defined grace as
... not a substance that exists by itself, or even apart from the soul; therefore it is a physical accident inhering in the soul ... Sanctifying grace may be philosophically termed a 'permanent, supernatural quality of the soul'"2
The Eastern Orthodox view, on the other hand, has been summarized by theologian Vladimir Lossky (1903-1958):
For Eastern tradition the created supernatural has no existence. That which Western theology calls the supernatural signifies for the East Uncreated - the Divine Energies ineffably distinct from the Essence of God. The difference consists in the fact that the Western conception of grace implies the idea of causality, grace being represented as an effect of the Divine Cause, exactly as in the act of creation; while for Eastern theology there is a natural procession, the Energies shining forth eternally from the Divine Essence. It is in creation alone that God acts as a cause, in producing a new subject called to participate in the Divine fullness; preserving it, saving it, granting Grace to it, and guiding it towards its final goal. In the Energies He is, He exists, He eternally manifests Himself.3
Eastern vs. Western understanding of the consequence of the Fall
These divergences in the understanding of grace and free will lead to completely different notions of how the Fall in the Garden impacted man's posterity.
In the Western understanding, it could be said that man's nature remained the same after the fall as before the fall, but that he was impaired by the privation of sanctifying grace.
By contrast, the Eastern understanding is not this grace itself was somehow taken away, but rather that man's nature itself had become corrupted to such an extent that grace could no longer dwell within it to the extent previously. "The deprivation of Grace is not the cause," wrote Lossky, "but rather the consequence of the decadence of our nature."4
I believe that practically any aspect of theology can be extrapolated from these foundational beliefs of grace and ancestral sin.
I think it is fair to say that the focus of Western Christianity is largely how to "get back" the sanctifying grace that was lost at the fall. Roman Catholicism and Protestantism may disagree on how this is to be accomplished, but I think it is fair to say that they share the same goal.
The focus of Eastern Christianity is essence appears to be the same. By contrast, however, the emphasis is not on how to "get God to give it back", so to speak, but rather on what one must do to one's self in order for the grace that is ever-present to return. Hence within Orthodoxy there is a strong emphasis on synergy (i.e. cooperation of man's free will with the will of God); and on askesis - self-abnegation that shifts the individual's focus from self to God and others. Salvation within Western Christianity lies mainly in restitution. Within Eastern Christianity it lies principally in restoration.
Although Eastern Orthodoxy is often thought to have close affinity with Roman Catholicism due to its sacramental and liturgical tradition, at the very core of their theologies, Roman Catholicism and Protestantism have much more in common with each other than either do with Eastern Orthodoxy.
Returning to the relation between original sin and the Immaculate Conception, in the Orthodox understanding there never was any privation of grace in the first place, as the Roman Catholic Church understands it. The Immaculate Conception becomes, then, in the words of former Metropolitan Jonah of the Orthodox Church in America, "an unnecessary solution to a nonexistent problem."
1. Orthodox Dogmatic Theology (3rd ed.; tr. from Russian, St. Herman of Alaska Press, 2005), pp.193-194.
2. Vol. 6, p.705
3. The Mystical Theology of the Eastern Church (tr. from French; St. Vladimir's Seminary Press), pp.88-89