Blood on the Door
God instructed Moshe (Ex. 12:13) to command the Jewish people to place the blood from their Passover sacrifices on their doorposts to serve as a sign so that He would pass over their houses without harming them. As God clearly knew who was in each house, why was the blood necessary?
Rabbeinu Bechaye posits that the blood that the Jews placed on their doorposts wasn't some magical sign which intrinsically protected their homes from the plague. Rather, the blood symbolically demonstrated the Jews' faith that God would protect and redeem them. They trusted God to the point that they were willing to sacrifice one of the deities (a sheep) of their Egyptian masters and publicly display it without any fear of retribution. It was this unequivocal demonstration of faith which provided the merit for their protection and salvation.
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SAVED FROM DARKNESS
Rashi writes (Ex. 12:6) that when the time came for God to fulfill the vow that He swore to Avraham to redeem his descendants, He saw that the Jewish people didn't have any mitzvot to perform to merit their redemption, so He gave them the mitzvot of circumcising the males and of offering and eating the Pesach-sacrifice. If the time came for God to keep His promise, why didn't He have to fulfill it even if the Jews didn't have sufficient merits?
Rav Aharon Leib Shteinman, in Ayeles HaShachar, explains that although God certainly would have fulfilled His promise to redeem the Jews at this time, there are many possible forms of redemption. Rashi writes (Ex. 13:18) that only one out of five Jews merited being saved from Egypt, with the other four-fifths dying during the plague of darkness. Had it not been for the additional mitzvot that God gave the Jews, perhaps an even smaller number of Jews would have been redeemed in fulfillment of God's promise.
Additionally, other miracles of the redemption, such as the level that they reached in seeing the Divine presence at the Red Sea weren't included in His promise and were only received through their actual merits.
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NEXT TO LAST
The Tosefos Yom Tov writes (Demai 7:3) that some people ask a powerful question based on the prophetic verse in Chaggai 2:9: "the glory and honor of the last Beit Hamikdash will be even greater than that of the first." This verse is referring to the Second Holy Temple in Jerusalem, which was destroyed almost 2,000 years ago. In referring to it as the "last" one, it seemingly indicates that there won't, God forbid, be another Temple ever built.
The Tosefos Yom Tov answers that many times the word "last" doesn't mean the final one. Rather, it refers to the last one vis-a-vis the first one, even though there may indeed be others that come after it. Although this sounds a bit foreign grammatically, he cites two places where the Torah uses such language. One is in Exodus 4:8-9 (the other is in Genesis 33:2), in which God tells Moshe that if the Jews won't believe the first sign, they will trust in the last sign. God adds that if they won't believe the "last" sign, they will surely believe the third one in which Moshe will turn the water of the river into blood.
The Kehillas Yitzchok brings a clever hint to this proof from our parsha (Exodus 12:13) "vehayah hadam lachem l'ot al havatim," which literally means that the blood of the Passover-sacrifice will be a sign on the doors for God to skip over that house. However, it can also be understood as stating that the blood (which was the third proof of Moshe's legitimacy) will be a sign for you regarding the Temples, as if anybody attempts to prove from Chaggai 2:9 that the second Temple was the final one, we may now answer that the blood mentioned in our verse proves that it isn't so.
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THE CHAMETZ EXILE
The Midrash teaches (Eichah Rabbasi 1:28) that the Jews were punished and sent into exile for eating chametz on Pesach. Where is there any hint in the Torah that this transgression is punishable with national exile?
The Vilna Gaon (Genuzos HaGra) brilliantly points out that in the entire Torah, the word venichreta, which means that a sin is punishable by karet, always has the same cantillation, which is called tavir - broken. This alludes to the fact that somebody who commits such a sin will be spiritually broken and cut off from God. There is one exception. On the verse (Ex. 12:15) which says that any person who eats chametz on Pesach will be spiritually cut off - the cantillation on the word "venichreta" is called גרשיים - sending away This alludes to the fact that in contrast to every other sin which is punishable by karet, the punishment for eating chametz on Pesach is geirushin - exile and spiritual divorce.