geulah is a chok that defies reason

 מִשְׁכוּ וּקְחוּ לָכֶם צֹאן לְמִשְׁפְּחֹתֵיכֶם וְשַׁחֲטוּ הַפֶּסַח

Rashi comments משכו ידיכם מעבודה זרה וקחו לכם צאן של מצוה

R' Moshe Tzvi Neriah writes that you can learn two lessons from this Rashi.  We learn from Rashi 1) how difficult it is to break old habits and resist temptation, as a person can be on the cusp of geulah and still require warnings and admonishments not to engage in idolatry; 2) and we also learn that even though a person might still be steeped in and tempted by avodah zarah, he can still be worthy of redemption.

It's worth noting (see hesber of Shem m'Shmuel 5675) that when Hashem gave Moshe the command to offer korban Pesach, He said only וְיִקְח֣וּ לָהֶ֗ם אִ֛ישׁ שֶׂ֥ה לְבֵית־אָבֹ֖ת שֶׂ֥ה לַבָּֽיִת׃.  It is Moshe who added  מִשְׁכוּ, meaning משכו ידיכם מעבודה זרה, when he transmitted it to Bnei Yisrael.  

וּפָסַ֤ח ה׳ עַל־הַפֶּ֔תַח.  We know that Hashem says "pischu li pesach shel machat," just give me an opening the size of the eye of a needle, let me into your heart just a little bit, and I will open gates for you that you can drive an 18 wheel truck through.  Sometimes, though, even an opening the size of the eye of a needle is too much to ask for.  וּפָסַ֤ח ה׳ עַל־הַפֶּ֔תַח, Hashem overlooked the need even for that smallest opening.  Hashem himself did not ask for the משכו ידיכם מעבודה זרה.  This was the amazing thing about the geulah from Egypt.  It is moments before geulah and Moshe has to still tell people to give up their idolatry -- they still were not 100% committed to Hashem or bust -- and still, Hashem redeemed us.

After this command to take the korban and shecht it, the Torah goes on to speak about sprinkling the blood on the doorposts to protect against the malach ha'mashchis, and then ends the section  וּשְׁמַרְתֶּ֖ם אֶת־הַדָּבָ֣ר הַזֶּ֑ה לְחׇק־לְךָ֥ וּלְבָנֶ֖יךָ עַד־עוֹלָֽם.  Ramban and Ibn Ezra are bothered by the fact that this sprinking of the blood on the doorpost is not a mitzvah l'doros.  Where is the חׇק־לְךָ֥ וּלְבָנֶ֖יךָ עַד־עוֹלָֽם?  

(R' Berel Povarsky in his Bad Kodesh writes that there are 2 dinim in the zerikas ha'dam on the doorposts: 1) a unique din by pesach Mitzrayim that there had to be blood there to protect the house; 2) a din zerikas ha'dam like any other korban, as the doorpost was a substitute for the mizbeiach.  Nafka minah: if a person had multiple dwellings, then m'din zerikas ha'dam of the korban, he fulfilled the mitzvah as soon as he put the blood one one house and the korban is became permissible to eat, but m'din having blood on each doorpost of a home that needed protection, he needed to paint the doorposts of the other homes as well.  If so, maybe שְׁמַרְתֶּ֖ם אֶת־הַדָּבָ֣ר הַזֶּ֑ה לְחׇק־לְךָ֥ וּלְבָנֶ֖יךָ עַד־עוֹלָֽם is talking about the regular din zerikas ha'dam that applies l'doros by other korbanos.  Still a bit of a dochak.)

Ramban answers that you have to explain that this concluding pasuk is speaking only about taking and shechting the korban that had appeared earlier.  R' Shimon Sofer, however, explains that it is talking about the sprinkling of the blood.  "Halalu ovzei avodah zarah v'halalu ovdei avodah zarah," Bnei Yisrael had no merits to speak of to earn geulah.  How does a little blood on the doorpost warrant protection from the malach ha'mashchis when you have nothing else going for you?  The answer is that that's the  חׇק־לְךָ֥ וּלְבָנֶ֖יךָ עַד־עוֹלָֽם.  Geulah is a chok without reason.  There is no logic to explain how a person can still need a warning not to be an oveid avodah zarah and merit geulah, but that's how Hashem decided to make things happen.

There is perhaps another element as to why Moshe added the word מִשְׁכוּ here, which also connects to the idea of this parsha being לְחׇק־לְךָ֥ וּלְבָנֶ֖יךָ עַד־עוֹלָֽם something that should be transmitted for eternity.  Netziv comments as follows:

אבל לפי הפשט, באשר המצוה בפרשה הקודמת נאמרה לכל ישראל ליקח שה לבית אב, והיתה הדעת נותנת שיטפלו בזה האנשים הפשוטים בבית אב מי שרגיל למשוך טלה לשחיטה ולהפשיט, ולא מי שהוא גדול בבית אב ואין עסקו בכך, על כן קרא משה לזקני הדור והזהירם ביחוד ״משכו״ — אתם ״וקחו לכם צאן למשפחותיכם״ — בשביל כל המשפחה תהיו אתם המתעסקים בזה.

Had we just been given Hashem's command, we would have assumed that it should be the shleppers who do menial work who should go out and get the sheep and slaughter and butcher them.  Shlucho shel adam k'moso, so let them do the dirty work for us.  Moshe therefore added in his instructions מִשְׁכוּ וּקְחוּ לָכֶם, this is something you have to do for yourselves, not leave to the shleppers.  The mitzvah needs your hands-on from the first steps.

Why this should be the case is what the Torah is telling us in that last pasuk of שְׁמַרְתֶּ֖ם אֶת־הַדָּבָ֣ר הַזֶּ֑ה לְחׇק־לְךָ֥ וּלְבָנֶ֖יךָ עַד־עוֹלָֽם. No one in the household pays much attention to what the shlepper does, what the maid does, what the nanny does.  They are there to free up our time so we can carry on with the important things of life.  The Torah is telling us that if you want a mitzvah to be something your children take note of, something they will pass on to their children, then you can't delegate it -- you have to treat it like one of those important things in life, things that you make sure to take care of yourself.