What is the significance of the fact that Hashem sanctified the Levi?im in Egypt? Why did He do it?
1.
Seforno: When Hashem killed the firstborn of the Egyptians, the firstborn of Yisrael ? who were held responsible for the sins of the generation ? were not worthy of being spared;
1
so He sanctified them, thereby prohibiting them to do Avodas Hedyot
2
- and to require Pidyon.
3
Oznayim la'Torah: It implies that the Kedushah of the Bechorim is permanent - which explains they still need to be redeemed - and it is only the Avodah that was taken away from them.
3.
Bechoros, 5a: It implies that they continued to sanctify the Bechoros in the desert - even after they had redeemed the current Bechoros,
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and did not stop for the remainder of their stay in the desert.
2
Seforno: To stress that when He rejected the Bechoros it was not because He had changed, but because they had changed, and similarly, when He instructed that the Levi'im of the current generation should redeem the Bechorim and not in futue generations it is because He foresaw that the Levi'im of that generation were worthy but not the Levi'im of future generations - and not because He had changed His mind.