In the Nicene Creed of faith our Church is described as the "One, Holy Catholic and Apostolic Church": "One" because there can only be one true Church with one head Who is Christ... Each of these titles is limiting in some respects, since they define Christians belonging to particular historical or regional Churches of the Orthodox communion... After the seventh Ecumenical Council in AD 787, the basic unity of faith and ecclesiastical life between East and West began to disintegrate, due to a variety of theological, jurisdictional, cultural and political differences. This eventually led to the Great Schism between East and West of AD 1054. (http://www.goarch.org/archdiocese/)
And despite their communion, there are significant things that divide them somewhat:
► The Orthodox Church opposes the Roman doctrines of universal papal jurisdiction, papal infallibility, purgatory, and the Immaculate Conception precisely because they are untraditional. Clark Carlton, THE WAY: What Every Protestant Should Know About the Orthodox Church, 1997, p 135.
► The Orthodox Church does not believe in purgatory (a place of purging), that is, the inter-mediate state after death in which the souls of the saved (those who have not received temporal punishment for their sins) are purified of all taint preparatory to entering into Heaven, where every soul is perfect and fit to see God.
Also, the Orthodox Church does not believe in indulgences as remissions from purgatoral punishment. Both purgatory and indulgences are inter-corrolated theories, unwitnessed in the Bible or in the Ancient Church, and when they were enforced and applied they brought about evil practices at the expense of the prevailing Truths of the Church. If Almighty God in His merciful loving-kindness changes the dreadful situation of the sinner, it is unknown to the Church of Christ. The Church lived for fifteen hundred years without such a theory. — http://www.goarch.org/ourfaith/ourfaith7076
► Father Theodore Pulcini: What I found most disturbing in my reading was that the Orthodox objected to the doctrine [of the Immaculate Conception] not so much because of its proclamation of Mary as immaculate (indeed, the Orthodox liturgy repeatedly refers to Mary as "all holy ... .. immaculate," and "most blessed") but because of the erroneous understanding of original sin underlying it...
I sadly concluded that the erroneous Roman understanding of original sin had led to another erroneous teaching, the dogma of the Immaculate Conception. The dogma was clearly an unwarranted innovation.
It was much the same with the dogma of papal infallibility. This doctrine asserts that when the pope speaks ex cathedra, "from the throne," or officially, on matters of faith and morals, he teaches infallibly. Thus the whole Church is bound by his teaching. Orthodoxy and Catholicism - What are the differences - Father Theodore Pulcini ISBN 978-1-888212-23-5 [69] http://almoutran.com/2011/03/251
► Roman Catholicism, unable to show a continuity of faith and in order to justify new doctrine, erected in the last century, a theory of "doctrinal development."
Following the philosophical spirit of the time (and the lead of Cardinal Henry Newman), Roman Catholic theologians began to define and teach the idea that Christ only gave us an "original deposit" of faith, a "seed," which grew and matured through the centuries. The Holy Spirit, they said, amplified the Christian Faith as the Church moved into new circumstances and acquired other needs....
On this basis, theories such as the dogmas of "papal infallibility" and "the immaculate conception" of the Virgin Mary (about which we will say more) are justifiably presented to the Faithful as necessary to their salvation. http://www.ocf.org/OrthodoxPage/reading/ortho_cath.html
► It was the charge of the Reformers that the Catholic doctrines were not primitive, and their pretension was to revert to antiquity. But the appeal to antiquity is both a treason and a heresy. It is a treason because it rejects the Divine voice of the Church at this hour, and a heresy because it denies that voice to be Divine...
I may say in strict truth that the Church has no antiquity. It rests upon its own supernatural and perpetual consciousness. Its past is present with it, for both are one to a mind which is immutable. Primitive and modern are predicates, not of truth, but of ourselves. — Most Rev. Dr. Henry Edward Cardinal Manning, Lord Archbishop of Westminster, The Temporal Mission of the Holy Ghost: Or Reason and Revelation (New York: J.P. Kenedy & Sons, originally written 1865, reprinted with no date), pp. 227-228.
Thus you have devotees on both sides warning about ecumenism:
► Then there are those who attempt to join together all Christian religions into one faith. They would be horrified at the idea of a service with Hindus and Christians celebrating together, yet they do not bat an eyelash at the idea of Orthodox celebrating with Roman Catholics, who with no authority broke off from the Church close to a thousand years ago. (http://www.orthodox.net/articles/against-ecumenism.html)
► Few Catholics realize that Eastern Orthodoxy, especially as represented by Palamite theology, represents a systematic and comprehensive attack upon Catholic doctrine. Catholic and Orthodox theology are not only in opposition to one another in their understanding of God (theology), but also in the various disciplines of philosophy – in Cosmology, Psychology, Epistemology, Metaphysics, Theodicy, and Ethics. They posit radically different views of God, of man, and of the relationship between God and His creation. Finally, and very crucially, they embrace radically different views of the final destiny of man. In this respect they both employ the concept of "deification", but possess very different understandings of what this term signifies. http://www.waragainstbeing.com/partiii
More on differences http://www.examiner.com/orthodox-christian-in-roanoke/orthodoxy-101-comparison;
http://turretinfan.blogspot.com/2009/12/lay-roman-catholic-and-eastern-orthodox.htm:
See also Disagreements under different models of supreme authority and http://www.ocf.org/OrthodoxPage/reading/ortho_cath.html http://orthodoxinfo.com/inquirers
Eastern Christians do not care much for Augustine’s theology, which means that the gap between them and Protestants is very wide. To conclude there is a radical contradiction between Catholics and Orthodox is to ignore the similarities. Bonaventura is as “spiritual,” as any eastern sage, and a modern Catholic saint, Padre Pio, as mystical as any eastern monk. As for Mary, the eastern respect for theotokos is very deep.