Q: Why do the Orthodox use leavened bread since leaven is a symbol of sin? Is not Christ’s Body sinless?A: Orthodox Christians use leavened bread because this is the ancient apostolic practice. The introduction of unleavened bread dates to the 8th century. As the Catholic Encyclopedia notes: “It is a debated question whether Christ used leavened or unleavened bread at the institution of the Holy Eucharist, since different conclusions may be drawn, on the one hand, from the gospel of St. John and the synoptic Gospels on the other. History does not establish conclusively what the practice of the Apostles and their successors was, but it may be asserted with some probability that they made use of whatever bread was at hand, whether azymous or fermented.” Most Orthodox scholars would argue that St. John’s chronology should be seen as a clarification and therefore adhered to. This, combined with the fact that the word ‘artos’ generally means regular bread, favor the view that the Lord used leavened bread.
It is true that yeast is often presented in Scripture as a symbol of sin, but as common bread was used throughout the year for the Eucharist in the Apostolic Churches, the leavened character was understood as symbolic of life – the living and risen body of Christ. (See also Luke 13:20).
Passover used unleavened bread exclusively, which is the bread Jesus used at the Last Supper.He was celebrating Passover when he instituted Communion.