Notice, the verse tells Moses to Take the Levites because the firstborn are holy. That doesn’t make sense.
Shouldn’t it say take the Levites because the Levites are holy or it should saythe firstborn are holy, so take them?
Surely, it does not make sense to take the tribe of Levi for God because the firstborns were saved in Egypt!
Sources: Num. 3: 12, Bemidbar Rabbah 3:5