Here is a good article on the topic. I would post the url, but I'm such a Newbie that I can't. It explains the topic much better than I ever could.
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Holy Communion and the Church
Can we(Orthodox) receive Holy Communion in a Roman Catholic Church?
by Rev. Dr. Stanley S. Harakas
There is a very close relationship between Holy Communion and the Church. Baptism, Chrismation, and Holy Communion define us as members of the Church. In Baptism we die and are resurrected with Christ to a new existence, what Saint Paul calls a "new creation." He said, "Therefore, if any one is in Christ, he is a new creation; the old has passed away, behold, the new has come." (2 Corinthians 5:17) As soon as we are baptized, we are chrismated, that is, we are anointed with the "seal of the gift of the Holy Spirit," and with that we assume our place as laypersons in the Church. Immediately following that, we manifest our membership in Christ and the Church by receiving Holy Communion.
Through Holy Communion we are not only united with Christ by receiving His Body and Blood, but also with all the other members of the membership in the Body of Christ, the Church. Saint Paul put it this way: "The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread." (1 Corinthians 10:16)
The next step to understand is that one of the ways the Church disciplines its members for violations of the Christian way of life is to "excommunicate" a member, for either a limited, life-long or permanent time. Excommunication means that the member may not receive Holy Communion. A person who has sinned grievously, for example, may be prohibited from receiving Holy Communion for a year or more. A person who has entered into a marriage outside the Church is prohibited from receiving Holy Communion until his or her situation is regularized in some way. A person who is a schismatichaving separated himself from the canonical Bishopor who has been condemned for holding and promoting false teachings may be excommunicated per-manently. That is, the person is no longer a member of the Orthodox Church.
This same practice occurs when groups, large or small, separate themselves from the Church. This occurs when there is a division, such as occured about a thousand years ago, between the Orthodox and the Roman Catholics. Because we are divided on issues of faith and order, there is no sharing of Holy Communion. Until steps are taken that will make it possible for the Orthodox and the Roman Catholics to overcome their differences on issues of faith and order, it is inconsistent and contradictory for us to share in each others Holy Communion. Sharing in Holy Communion means that these obstacles have been overcome, and that therefore we are united in faith and order. Holy Communion, then, manifests that union. That is why the Orthodox Church is in ecumenical dialogue with the Roman Catholic Church, working toward a legitimate and theologically proper reunion. Sharing in Holy Communion is not the means to reunion, it is the end result of reunion.
A Difference About Our Differences
What Ive outlined above is the position of the Orthodox Church, which sees the differences between Orthodoxy and Roman Catholicism as dealing with both doctrine (the Filioque) and church order (the Pope). However, while the Roman Catholic Church acknowledges our division, they hold that the division is only one of church order. Consequently, they feel that the division is less severe than the Orthodox do. As a result, since their Vatican II Council (1962- 1965) they have approved that Orthodox Christians may receive Holy Communion in their parishes under certain circumstances.
The Roman Catholic Church and the Orthodox Church hold essentially the same teaching about the relationship of Holy Communion and Church membership. The difference is that the Roman Catholic Church sees their differences with the Orthodox Church as consisting of secondary features, that do not break the unity of faith with them. The Orthodox, however, understand their differences as including substantial differences on issues of doctrine and beliefs. These will need to be overcome, as far as the Orthodox are concerned, before the time can come so that we can implement our unity in faith, by sharing in Holy Communion as members of one re-united Church.
Conclusion and Practice
So, until that takes place, you ought not receive Holy Communion in a Roman Catholic Church. As an Orthodox Christian, among the many things that happen when you receive Holy Communion, is that you manifest your identity as an Orthodox Christian and affirm your membership in the Orthodox Church.
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