My Parents’ former Presbyterian Pastor converted to Eastern Orthodoxy. They use Leavened Bread, symbolizing Christ’s rising from the dead.
Catholic Christians, however use unleavened bread for a couple of reasons. One, because Jesus used unleavened bread when he instituted the Eucharist. Second, because Jesus referenced the pride of the Jewish leaders as being puffed-up like leavened bread (Matthew 16:6, Luke 12:1, 1 Corinthians 5:6-8). Jesus humbles Himself in the Eucharist. He does not come as someone who is puffed-up, though He deserves the greatest of reverence and devotion.
Though both are beautiful in their symbolism, the Eucharist is not merely symbolic, but is truly the body, blood, soul, and divinity of Jesus Christ, our Savior! This gift is true food and true drink (John 6:55).
Just as it is better for a baby to have his mother’s milk than it is to have something that symbolizes his mother’s milk for physical nourishment, it too is better for us to receive the real gift than something that is merely symbolic for spiritual nourishment.
Keeping with this theme of a baby, some Protestants (#notAll) will mock ancient Christians for believing in the Eucharist, but these same individuals might say (as we do) that this baby born in the manger is God, though this greatest gift to many looks like a baby.