“In place of every firstborn, first issue of the womb from the children of Israel” – initially the service was performed by the firstborns. Because [the firstborns] sinned with the calf, the Levites merited to take their place because they did not err with the calf. “For every firstborn is Mine…” – just as it says: “Sanctify for Me every firstborn…” (Exodus 13:2), “They shall be Mine, I am the Lord.”
Because it says: “I have hereby taken the Levites from among the children of Israel in place of every firstborn,” do I hear that from this day forward the firstborn are no longer sacred? The verse states: “They shall be”; it teaches that they require redemption.17Even though the firstborns were replaced by the Levites, they are still holy and they must be redeemed by giving five Shekalim to a priest.