Eastern Orthodox/theological importance
what is the theological importance of leavened and the unleavened bread in the eastern orthodox church?
There is a very detailed explanation at this site:
To summarize, leavening represents a connection with the past, a flavoring when it becomes old, a force that invisibly spreads, a change that is also unseen, complexity, activity/festivity and involves labor. Dough without leaven represents haste, a break with the past, an absence of extra flavor, simplicity, inactivity, powerlessness and a lack of labor.
It is the Lord who brought Israel out of Egypt, not a work of the people. To remember this they eat unleavened bread as a symbol of their powerlessness. They also ate the unleavened bread because they left Egypt in a rush. The Passover, which is eaten with bitter herbs, is not in any sense a feast or celebration, but rather a ceremonial recollection of the power of God. Unleavened bread is even referred to as the ‘bread of affliction’ in De 16:3, recalling the haste in which Israel fled Egypt. This haste of the flight was important: it revealed that the people had not plotted it but that God did it all on His own. They also remember the affliction they suffered in the land of the Egyptians. It is also important to remember that unleavened bread was also mandated only for the seven days of the Passover (Ex 12:15). It is not an indefinite commandment, but one limited to just this particular commemoration.
The Bible says that leavened bread was almost never to be involved with sacrifices (c.f. Ex 29:23, Le 8, Nu 6:15-19). There is only one time when leavened bread was offered, to represent the thanksgiving of the people:
Le 7: 13 Besides the cakes, he shall offer for his offering leavened bread with the sacrifice of thanksgiving of his peace offerings.
Le 23:17 Ye shall bring out of your habitations two wave loaves of two tenth deals: they shall be of fine flour; they shall be baken with leaven; they are the firstfruits unto the LORD.
So leaven represents the works of the people, which they offer to God with thanksgiving.
As for leaven and leavened bread, it is used two ways in the Gospels. The first is to denote the power of the Kingdom of Heaven.
Mt 13:33 Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.
Lk 13:20 And again he said, Whereunto shall I liken the kingdom of God? 21 It is like leaven, which a woman took and hid in three measures of meal, till the whole was leavened.
The Kingdom spreads like yeast! Think of it: there is no outside change when one repents and becomes a Christian, yet it somehow changes not just individuals but the entire world. We can now look back on history to see how Christianity changed so many people and see the truth here.