After informing the Jews to perform the mitzva of the Korban Pesach, to protect from the final plage, the Torah tells us how וַיֵּלְכוּ וַיַּעֲשׂוּ בְּנֵי יִשְׂרָאֵל כַּאֲשֶׁר צִוָּה ה’ אֶת מֹשֶׁה וְאַהֲרֹן כֵּן עָשׂוּ – The Jews went and did as the Lord commanded Moses and Aaron, and so they did. (12:28)
It is quite perplexing as to why the Torah would insert כֵּן עָשׂוּ, which Rashi explains to mean that Moshe and Ahron also performed the mitzva. Would we have expected otherwise?
But why is human input necessary at all? The first nine did not affect Jews, by default. For example, frogs, animals, and hail did not enter Jewish areas. Why by the final plague is there a requirement to perform this Mitzva and smear the door posts and lintel in order to be saved?
Furthermore, the Korban Pesach was not the only Mitzva given that night; circumcision was instituted that night too. What is particularly special about these two mitzvos that they needed to be instructed to perform them on the night of the Slaying of the Firstborn?
In Ezekiel (16:4) the Prophet says that the Jewish People were “born” in Egypt. There are two aspects of the Jewish People – the “doing”, and the “being”, active and passive.
Every person born to a Jewish mother is a Jew from the moment they open their eyes until the moment they finally close them. This is passive, the “being” aspect. There is not a thing that can change this; people can convert or practice other religions, but halacha states that they remain Jews, they can just opt right back in. There is no element of choice in the matter. G-d chose the Jews, and that choice obviously cannot be undone.
The second aspect, of “doing”, is much more down to personal choice, to do as we are told.
Circumcision is a passive mitzva – it is performed 8 days after a boy is born and the child will have no knowledge or choice in the matter. It cannot be undone either. However, Korban Pesach is a personal choice, an active mitzva to be done. It is the only mitzva in the entire Torah where the word עבודה, service, is used in terms of a mitzva. It is not for nothing that these are the only two positive commandments whose punishment for not performing them is Kareis, spiritual excision – being cut off from Hashem.
R’ Shlomo Farhi observes how in Parshas Yisro, Moshe Rabbeinu is described as an Egyptian man. Moshe Rabbeinu! The same would be true of the Jews in Egypt, themselves almost Egyptian. These mitzvos set them apart.
So why did the Jews have to do something to be saved from the 10th plague? R Yehoshua Hartman explaining the Maharal teaches that the difference here is that Hasehm performed it Himself. Hashem says this time, it will be Him; not an angel, messenger, seraph etc. This has an obvious implication that with the other 9, angels and messengers were used. The Maharal explains that angels are not usually able to harm Jews, as Jews are a higher spiritual being. But Hashem is not subject to this instruction; He is above everything! He was looking for people to bind themselves to Him through the Mitzvos He gave them; it wasn’t like the other nine where it had been enough to be born to a Jew. They had to demonstrably show in a visible way they identifed themselves with G-d to be saved, with these mitzvos.
With this in mind, we can answer the first question. If people had to prove they were with G-d, one might think that being G-d’s spokesperson to Egypt and the Jews was enough for Moshe and Ahron; that they’d shown who their lot was cast with. This is inaccurate. Moshe and Ahron only did what G-d asked them when they were asked to fulfil a task, but whenever they weren’t in His service, they were just normal people. So the pasuk said כֵּן עָשׂוּ to explain that actually, Moshe and Ahron also performed the Mitzva, and they too only became servants, עבדים of Hashem after performing the עבודה of the Korban Pesach.
The doing/being has another relevancy to us. The mitzvot of Tfilin וּקְשַׁרְתָּם לְאוֹת עַל-יָדֶךָ וְהָיוּ לְטֹטָפֹת בֵּין עֵינֶיךָ And you shall bind them as a sign upon your arm, and they shall be for frontlets between thine eyes. The brachos we make show this, one is “lehaniach” – on the binding, one is “al mitzvas” – passive, on the mitzva. This is further shown in the fact that if one puts on Tefilin before sunrise, one must retie the arm Tefilin to make the bracha, but does not need to adjust the head Tefilin. The reason for this is that our arms are what we use to do, so they must be active, and perform actions. Our heads are passive, our minds are who we are, and as such no new bracha is required.