민수기 8:17의 주석

כי לי כל בכור FOR ALL THE FIRSTBORN [OF THE CHILDREN OF ISRAEL] ARE MINE — This means: they became Mine by strict right (lit., by the line of justice), because I protected them distinguishing between them and the firstborn of Egypt, and I took them as Mine — until they sinned by worshipping the Golden Calf, but now ואקח את הלוים I HAVE TAKEN THE LEVITES in their stead.

כי לי כל בכור, I had already owned the firstborn whose duty it had been to perform the tasks now given to the Levites. The reason that the firstborn had originally been charged with the duties was that they were the most distinguished members of their respective families and one delegates those to represent one before one’s lord and master, before G’d.

Since I protected them. Rashi is answering the question: Why does Hashem say “for every firstborn [of Bnei Yisroel] is Mine” which implies that they are still His, when it is written (v. 18) “I have taken the Levites” which implies that He had already taken them. In response [to this question] Rashi explains that “they were Mine by the rules of justice” [meaning according to the strict interpretation of the law]. Re’m writes that with this he answers that “for every first born is Mine” does not refer to the present, because they were disqualified when they erred with the golden calf. It merely means that they were His initially. Rashi also informs one of the reasons why they were His by the rules of justice — “because I protected them…” Rashi states “I took them for Myself” instead of “I have sanctified them [to be mine]” in order to correspond with “I have taken the Levites instead of all firstborn” (v. 18) which means I have now taken them for Myself in the place of those whom I had taken before. He adds a vav on the word ולקחתי (and I have taken) which is in the place of הקדשתי (I have sanctified) in order to indicate that they are two different ideas — “for every firstborn is mine … on the day in which I slew…” and also “I have sanctified them” at that time for my service. However, once they sinned they were disqualified and I took the Levites instead of them. Rashi concluded “now I have taken” so that you should not say “I took them” (v. 18) refers to the time that I sanctified the firstborn for Myself. For it is written, “instead of all firstborn” which implies that they had left the service and the others entered in their place.

ביום הכותי כל בכור, “on the day when I smote every firstborn. On a different occasion, in Exodus 12,29, the smiting of the firstborn was not described as having occurred on “a day,” but as having occurred at midnight. We see from here that every time the Torah uses the term: יום, “day,” it speaks of a 24 hour period, i.e. night and day.

ביום הכותי כל בכור בארץ מצרים הקדשתי, the reason why I required them now to undergo the procedure of being redeemed from their sanctity and the duties that were part of this that from the day I had sanctified them, was that they were not supposed to engage in mundane activities, such as earning a livelihood, at all.
Firstborn male animals that are acceptable as sacrifices on the altar may not be used for mundane purposes, i.e their owners must neither let them perform work, not shear their wool and use it for themselves. This was in order for Me to have demonstrated their holy character. These regulations were imposed in order to justify these firstborn not being killed along the firstborn of the rest of the Egyptians and their herds. Having been part of the Egyptian culture, and having been the most distinguished in their respective families, they should, ordinarily, have become part of Psalms 78,49 calls the משלחת מלאכי רעים, “the wrath inflicted by the band of deadly messengers.” The surviving firstborn humans and their offspring, in order not to lead a life of idleness, therefore have to be “redeemed,” so that they can at least lead normal lives. G’d therefore accomplished this by the stratagem known as פדיון, redemption.