We read about the tribe of Levi in Parshat Bamidbar (Bamidbar 3:11-13):

God spoke to Moshe, saying: “Behold! I have taken the Leviim from among B’nai Yisrael, in place of every firstborn, the first issue of every womb among B’nai Yisrael, and the Leviim shall be Mine.  For every firstborn is Mine: On the day I struck down every firstborn in the land of Egypt I sanctified every firstborn in Israel for Myself, from man to beast; they shall be Mine- I am God.

Originally, the service in the Mishkan was supposed to be performed by the firstborn sons. However, when the firstborn sons participated in the sin of the golden calf, they lost the........

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