In order to ensure that they survive Makat Bechorot, Bnei Yisrael are given one very specific instruction: They must take blood from the newly-slaughtered Korban Pesach and smear it on their doorposts. When God sees this blood, He will pass over that house, sparing its firstborn from death. This is a cool-sounding practice, but why is it necessary? Does God need an external sign to differentiate between Bnei Yisrael and Egyptians?
To explain this strange command, we must analyze the reason Egypt enslaved Bnei Yisrael in the first place. In the beginning of Sefer Shemot, Pharaoh explains, “VeHayah Ki Tikrenah Milchamah VeNosaf Gam Hu Al Sone’einu, VeNilcham Banu,” “If a war occurs, [Bnei Yisrael] will ally with our enemies and fight against us” (Shemot 1:10). Essentially, Pharaoh fears that Bnei Yisrael are so distinct from the native Egyptian people that if they were given a choice, Bnei Yisrael would rebel against Egypt rather than supporting Egypt in a battle against foreign invaders.
To quell this fear, Pharaoh attempts to assimilate Bnei Yisrael into Egypt. He forces them to be slaves to himself, just as the other Egyptians are (see BeReishit 47:25). Bnei Yisrael’s forced labor, is, of course, far more arduous than that of the Egyptian people, but the concept is the same. Bnei Yisrael are no different.
Pharaoh orders the murder of the Jewish baby boys for a similar reason. He does not wish to wipe out Bnei Yisrael, but he does want to force a generation of Jewish women to marry Egyptians (see Ralbag Shemot 1:22 s.v. Col HaBein HaYilod), thereby jeopardizing Bnei Yisrael’s paternal traditions. With time, Pharaoh hopes that Bnei Yisrael will assimilate to the point that they will no longer pose a security risk. This is Pharaoh’s plan for Bnei Yisrael—to turn them into Egyptians.
Eighty years in, this plan seems at least partially successful. When Moshe and Aharon demand that Pharaoh send Bnei Yisrael to serve Hashem in the desert and Pharaoh responds by forcing the Bnei Yisrael to find their own straw for bricks, the Jewish taskmasters criticize Moshe and Aharon, “Asher Hivashtem Et Reicheinu BeEinei Pharaoh UVEinei Avadav,” “For you have made our scent loathsome in the eyes of Pharaoh and his servants” (Shemot 5:21). Though they have not yet fully assimilated, Bnei Yisrael have grown to covet the praise of the Egyptians.
The Midrash even says that Bnei Yisrael stop circumcising their children in order to “be like the Egyptians” (Shemot Rabbah 1:8).
This assimilation is the most important thing that must change in order for Bnei Yisrael to become God’s nation. This explains why the Torah emphasizes so heavily that the Ten Plagues affect the Egyptians but not Bnei Yisrael (see Shemot 8:18-19, 9:4, 9:6-7, 9:26, 10:23, and 11:7). Hashem wants to stress to Bnei Yisrael that they are a nation distinct from Egypt.
The commandment to put blood on their doorposts stems from anti-assimilation as well. At the end of the day, Hashem can stress the difference between Bnei Yisrael and the Egyptians over and over again, but unless Bnei Yisrael choose to accept that distinction, they can never be redeemed. The blood on the doorpost is a litmus test. By choosing to put blood on his doorposts, a man displays to everyone in the vicinity that he has rejected Egypt and declared himself part of Am Yisrael. If he refuses to do so, he is no different from the surrounding Egyptians and will share their fate.
In light of this, the species of animal whose blood is to be put on the doorposts—the sheep—now gains significance. Sheep are considered vile animals in Egypt (see BeReishit 46:34), and anyone who slaughters one puts himself in danger of being stoned by the Egyptians (Shemot 8:22). Therefore, the blood on the doorposts is a true challenge, a public affront to Egyptian culture. Only people who are legitimately confident in God’s ability to save them would demonstrate this daring devotion—and only those people deserve to be saved.
God could decide Himself which people will leave Egypt, passing over those who are worthy and leaving the rest to rot in slavery. But He doesn’t. Hashem recognizes that if a man is forced into Am Yisrael, he will never truly view himself as part of the nation. Such a momentous decision will only be lasting and meaningful if the man chooses to join of his own free will.
In our lives, we may never face a stark all-or-nothing test like the blood on the doorposts, but we are constantly subjected to choices regarding our Avodat Hashem. Whether to catch up on sleep or go to minyan on a lazy Sunday morning, whether to learn Torah or spend time with our families and friends, whether to improve ourselves or help others—the “right decision” is not always obvious, but we must recognize that our free will defines our lives, and we must be eternally conscious of the choices we make and their consequences. May we be Zocheh to fulfill the wise words of Moshe Rabbeinu (Devarim 30:19): “UVacharta BaChayim,” “choose life.”
 The Midrash Rabbah’s idea that Shevet Levi is not enslaved (Shemot Rabbah 5:16) fits very nicely into this framework, as the Egyptian priests are not enslaved either (see BeReishit 47:22).
 Technically, Korbanot Pesach could be sheep or goats, but Tosafot claim that historically, most Korbanot Pesach were sheep (Pesachim 3b s.v. VeAlyah), and the majority of Rishonim, including Rashi, Rashbam, and Ralbag, explain that when Moshe tells Pharaoh that Bnei Yisrael will sacrifice “To’avat Mitzrayim,” Moshe refers specifically to sheep (Shemot 8:22 s.v. To’avat Mitzrayim). Therefore, it seems reasonable to posit that there is significance to sheep in this regard.