(The following ideas are taken from R' Avigdor Nevenzahl's book of essays on Parashat Shmot.)
“Ushmartem et hamatzot ki be’etzem hayom hazeh hotzeiti et tzivoteichem me’eretz Mitzraim ushmartem et hayom hazeh ledoroteichem chukat olam… Shivat yamim se’or lo yimatze bevateichem ki kol ochel machmetzet venichreta hanefesh hahi me’adat Yisrael bager uve’ezrach ha’aretz” (Shmot -)
When taking a look at this week’s parasha, one can see that the isur of eating chametz on Pesach is more severe than any other mitzva on Pesach, and that matza takes a very central role in the Pesach experience. Upon further thought, though, this seems strange! We're focusing on what seems like such a small part of the ge'ula process- simple unleavened bread, as opposed to the multitudes of miracles that Hashem performed. Furthermore, the command to make matzot came two weeks before yetziat Mitzraim- all the more reason that it shouldn't be the ikar of the mitzva on Pesach!
We learn that in Mitzraim, Bnei Yisrael were at the 49th level of tuma- so tamei that they were almost never redeemed. What they ultimately achieved, though, was an irreversible, almost eternal, ge’ula- Bnei Yisrael will never return to the previous state of tuma that they had in Mitzraim. There's a pasuk "vehikeiti kol bechor bemitzraim- ani Hashem". In the hagada we say "ani- velo malach". Hashem Himself (as opposed to an angel/shaliach) had to come and perform makat bechorot since Jews and Egyptians were barely distinguishable from one another (in appearance, as well as practice). At Kriat Yam Suf, both the Jews and Egyptians were ovdei avoda zara, but only Egyptians died- because the Jews hadn't yet reached rock bottom (no pun intended).
What the isur of eating chametz comes to teach us is the difference of just a moment. The difference between the Jews being bestowed eternal Shechina versus never leaving Mitzraim was only a moment. Another minute in Mitzraim, Rav Nevenzahl writes, and BN"Y would have been goners- they would have been "machmitz". That's why matza is such a central idea in the ge'ula process- it represents that turning point when Bnei Yisrael left Mitzraim, and were redeemed through the ge'ula that they were so close to missing. If matza goes just a tad further than its allotted time, it becomes chametz. Chametz and matza have same letters- just a difference of a chet and a hei. If the hei of matza goes just an iota further, it becomes a chet- chametz.
We see that the clear and very fine border between tuma and tahara is a recurrent theme in Pesach. In Shmot 12:6, the criteria for Korban Pesach are described: “vehaya lachem lemishmeret ad arba’a asar yom lachodesh hazeh veshachatu oto kol kehal adat Yisrael bein ha’arbayim”. Various mefarshim try and clarify the idiom “bein ha’arbayim”. Rashi says this means from chatzot (midday) until dark. Ibn Ezra says it's a time frame of an hour and a third- from sunset to when no light is left. He explains, however, that it would be too hard to prepare and give the korban in such a short time so Chazal extended the time frame to start at chatzot.
The Ibn Ezra’s opinion, however, raises a fundamental question. Why would the Torah command us to do something that's too hard, only to be changed by Torah shebe'al peh? The answer is that Torah shebichtav is once again teaching us a crucial lesson- that there's a very thin line between tamei and tahor. The goal of commanding Korban Pesach was to separate Jews from Mitzrim; from tuma to avodat Hashem. There was very short time frame for Bnei Yisrael to declare their allegiance, so to speak, and if the korban wasn't performed within that hour and a third, then it was pasul. After that, Torah shebe'al peh came and amended it to make it practically more feasible- but the lesson is clear.
The line between tamei and tahor is very, very thin. Ultimately, Bnei Yisrael still had that spark in them, the spark that differentiated them from the Egyptians. That tiny difference was why Hashem distinguished between the Jews and the Mitzrim at yam suf and redeemed Bnei Yisrael. That's why even the smallest amount of chametz is asur- once the fine line between tamei and tahor is crossed, there’s no going back.
This is why matza plays such a central role in the Pesach experience. Matza represents so much more than its alimentation role in the leaving process. It represents the distinction between impure and pure, between eternal languishing and redemption, between avoda zara and avodat Hashem. Only once we understand this message, and apply it to our own avodat Hashem, can we truly internalize the Pesach experience.
May we all, through the paradigm of matza, strive to achieve that same “afeila le’ora”- from shibud Mitzraim to matan Torah, from nearly the lowest of lows to the highest of highs.