I offer some biblical and theological thoughts on the question of what kind of bread to use in the Lord's Supper. This is not a polished essay. I'm just "casting my bread upon the waters," as Solomon advises (Eccl. 11:1).
1. The main thing to think about is that the unleavened bread is bread made without the old sour "starter" dough that contains the yeast. In normal situations the yeast comes from that starter "leaven." "Leaven" refers to the old, sour reserved dough that contains the yeast. Leaven contains yeast. Leaven is not the same as yeast. The NIV and other translations screw this up entirely.
2. When bread is being made, there are two sources for yeast. First, you can get yeast from the old leaven that you have "reserved." In fact, the yeast must be imported into the "leaven" at the start of the process. When the process begins yeast is cultivated from the lees of wine. Then the yeast is put into the dough. A portion of the dough is used to cook the first batch, but a larger portion is "reserved" in order have the yeast readily available for use in later loaves. This "starter" dough that is kept is what is called the "leaven." It's the old, sour dough used as a delivery mechanism for the yeast.
3. The second way to get yeast is to get it "fresh" when it's cultivated from wine. In the ancient world people knew how to rise bread without the sourdough lump (leaven). Technically, a fresh loaf of bread made with this newly cultivated yeast is not "leavened." What this means is that the first batch of yeasted, rising bread that is made with the new yeast is not leavened bread. It is yeasted, but not from the sour dough leaven. So you could say that a loaf made with yeast not from the reserved, sour leaven is still unleavened bread. It's new. It doesn't use the old, sour stuff. The bread we use for the Lord's Supper in our church is not sour dough bread. It is not leavened bread. It contains fresh yeast. It is yeasted, but not leavened. This is theologically and symbolically significant.
4. There was theological symbolism involved in the use of unleavened bread when the Israelites made their flight from Egypt. They left behind the old, sour world of Egypt and began a new life. It was called "the bread of haste" because they had no time to cultivate new yeast. The process takes time. So they had to eat unleavened and un- yeasted crackers. Once they arrived in the promised land, they started over again with the yeast of God's new world. They would no longer get their "life" from Egypt, but now from the Lord's Spirit. They ate unleavened bread in the wilderness until they could get to the new land, harvest grapes, make wine, and also new yeast. Thus, the "old leaven" and the leavened bread made with it were associated with the old world, the old creation (1 Cor. 5). One cuts off Egyptian leaven and finds new yeast in the New Land. In between, one eats unleavened bread. The annual Feast of Unleavened Bread seems to be a week long return to the wilderness, the inbetween place. The association of yeast with the feast of Pentecost makes it clear that by that time new yeast has been found and is being used.
4-1/2. Of course, this does not mean that the word translated "leaven" or "yeast" in the Bible always refers to something negative. One has to understand the context and the story being told. Sometimes "leaven" is used to describe the delivery mechanism for the yeast (Matt. 13:33; Luke 13:21). At other times it's being used to describe something closer to the old, sour stuff that needs to be discarded (Matt. 16:6; Mark 8:15; 1 Cor. 5:6-8).
5. So the general question is whether in the New Covenant we must reenact the history of redemption weekly or annually, or whether that would be the same error as transubstantiationism? It seems to me that using unleavened bread for the Lord's Supper would be like doing animal sacrifices. It was part of what was done over and over until Jesus came and finished it. The new yeasted dough each year gradually fell back and became Egyptian sour leaven during the year and had to be cut off anew a Passover. But the New Covenant is ongoing, and cannot fall back, because it is guaranteed by the death of Jesus, not by the death of animals. The Spirit-yeast of Acts 2 does not fall back and become Egyptian.
6. Now, once the old leaven is thrown out and you are in haste to make bread, then it can be made without yeast. That apparently is what the Israelites did when they hurried out of Egypt. That's why that bread was called "the bread of haste." They left behind Egypt (the old leaven) and hurried out to cross the Red Sea. But there's nothing to suggest that we have to eat the bread of haste (unleavened and unyeasted bread) at the Lord's Table. In fact, Jesus reclined at table and we relax for the covenant memorial meal. The old leaven is gone, so we don't eat sour dough bread. But we do eat newly yeasted bread that represents Jesus new kingdom and the life he gives us at his Table.
7. You should note also that in Israel the bread eaten during the sacrifice of peace was leavened or yeasted. It is only Passover and the ensuing week that required unleavened bread alone be eaten, and that was not only during the special meal but at every meal for the week. After that, all the bread eaten, including at feasts and sacrifices, was yeasted and/or leavened. Yeasted bread was offered at Pentecost, and this newly yeasted leaven is thus associated with the coming of the Spirit, the divine Yeaster. We live after Pentecost. Check out Jack Collins's article "The Eucharist as Christian Sacrifice" in the Westminster Theological Journal where he argues that the Lord's Supper is the fulfillment of the sacrifice of peace (WTJ 66.1 [Spring 2004], pp. 1-23). The normal peace meal included yeasted bread. The one special one - Passover - did not in order to commemorate the Exodus and leaving the old Egyptian world (leaven) behind.
8. Oddly, we object to Romanists when they say (supposedly) that Jesus is in some sense re-crucified in the mass, but then by using unleavened bread we are really doing something similar: returning to Passover, to the Old Covenant, over and over again rather than worshipping in a post-Pentecostal way. Maybe that's pressing the analogy too much.
9. Remember, too, that the Lord's Supper is not the place for "afflicting ourselves" with bad tasting bread. There's an argument that relates the fasting that the Israelites did once a year with the Lord's Table. But that assumes, for one thing, that the Christian Table is simply an extensive of the Passover meal. It's not. But it also misses the fact that the Lord's Supper is a feast, not a fast. And the Table is most assuredly not the place in the Christian liturgy to "afflict ourselves" or "fast" from something that tastes good. The confession, mourning, affliction phase of the liturgy is at the beginning of the service. After the absolution we are forgiven and restored to happy fellowship with our heavenly Father. Making the Table into a place of mourning or affliction is just all wrong.
Besides, there's way too much fasting and mourning in the traditional church year. That has to be fixed some day. Now even Advent has become time of fasting. The Lord's Day is a day of joy and feasting. But in the Reformed world we are all about being "serious" about everything. The service has to be "serious" and the Supper has to be conducted with proper "seriousness." But why? When we get through the seriousness of confession and absolution we really don't want to keep coming back to our guilt and sin over and over again in the service, right?
The time for "seriousness" is at the beginning of the service when you confess your sins. Get serious then. Get it over with. On your knees. Confess your sins. Then rise and hear the declaration of forgiveness from the pastor. To dwell on your sin and guilt for the rest of the service is to fail to believe God has forgiven you in Christ.
People forget that we have a flow in the Divine Service. An order. A sequence. And that this sequence is psychologically satisfying, tuned to just what we need. The Supper is not ordinarily the place for seriousness. To think of it in terms of fasting is just weird. Here's an analogy that may help. What happens at the family table? The family dinner is the time to relax and enjoy one another, to eat and drink and be merry. If there's a need for "seriousness" in the family, it's dealt with before the supper in another room with a wooden spoon or a stern talk that ends with confession, forgiveness, and restoration of loving affection. When supper time comes, all that required "seriousness" has been dealt with. It should be the same way at the Lord's Table.
10. The Lord's Supper is not a continuation of the Passover. It's so much more than that. It's a fulfillment of all the covenant meals in the OT. Whenever God shared a meal with people it looked forward to the Lord's Supper and, of course, to the eschatological marriage Supper of the Lamb. To celebrate the Lord's Supper with unleavened crackers to reproduce the Passover is too anemic. The Lord's Supper is all the old world feasts and meals transformed and rolled into one ritual feast for the new people of God.
11. None of this is to say that using what is mistakenly called "unleavened" bread (crackers or matza) is evil or absolutely wrong. I just don't believe it's appropriate. I will say this as well: feasting on stale, tasteless "crackers" at the Lord's Table strikes me as less than desirable. We should be eating normal, tasty bread together and pleasant-tasting wine (or port) as well. Remember, unless you are using sourdough bread, normal bread is in fact "unleavened." But here's the bitter point: The atmosphere at the Table should be one of a friendly family meal. It's a table not a tomb. It's not a time to turn in on oneself and have an intensely private devotional experience. It's a common Table where we should acknowledge one another as members of the body of Christ. I believe using stale, hard crackers reinforces the mistaken notion that the Lord's Supper is a time of deep individual repentance and mourning for sins. That's just weird. That's not what happens normally at our dinner tables. And that's not what the climax of the Divine Service is about. Confession and forgiveness happened at the start of the service. The Supper at the end of the liturgy should be about relaxing, eating and drinking with the family of God, and enjoying our status as sons and daughters of the King of the universe.
Anyway, that's the basic argument.