Why do the Eastern Churches use leavened bread for communion? Why does the Roman Catholic Church use unleavened bread?
This difference is primarily cultural, although liturgical historians believe that there are theological reasons for the difference.
Originally, both the Roman Catholic Church and the Eastern Churches used leavened bread, leavened with yeast. Midway through the first millennium the practice of using unleavened bread became increasingly common in the Latin Church, until it became the general custom. This change was prompted by a desire to more closely associate the celebration of the Eucharist with Christ’s final Passover meal, the Last Supper.
The Eastern Churches, in contrast, preferred to continue using leavened bread. This is because in the Byzantine empire unleavened bread was associated with lifelessness, while the rising of leavened bread was associated with resurrection. There is one notable exception to this: the Armenian Church, which according to some sources used unleavened bread since the early centuries and continues to do so.
Unfortunately, this difference became a subject of hot controversy at the end of the first millennium, and was one of the factors behind the schism of 1054. Today this is no longer an issue, and everyone recognizes that both leavened and unleavened bread are capable of being consecrated as the Body of Christ.
When did the East/West split on the use of leavened/unleavened bread occur?
Most liturgical scholars believe that both the Eastern and Western Churches used leavened bread until the seventh century. However, the use of unleavened bread seems to be an ancient practice in the Armenian Church.
Is it true that in 1054, one of the reasons listed for the excommunication of the Eastern Orthodox Church from the Roman Catholic Church was that the Eastern Orthodox Church uses leavened bread for Holy Communion?
This was not one of the official reasons listed for the excommunication. Actually, it was the Patriarch of Constantinople, Michael Cerularius, who made a big fuss over the use of UNLEAVENED bread by Roman Catholics. No major authorities on the Latin side seriously attacked the Eastern use of leavened bread, although some minor polemicists may have.
Why does the Eastern Orthodox Church insist on the use of leavened bread?
In the Byzantine tradition the rising of the leavened bread is symbolic of the resurrection of Christ. Moreover, the use of leavened bread emphasizes that the Eucharist is something more than the Jewish Passover. As “Judaizing” Christian factions were a major problem throughout the East for quite some time, this was an important factor.