Charles Spurgeon Commentary Numbers 8:16-18

"For they are wholly given unto me from among the children of Israel; instead of all that openeth the womb, even the first-born of all the children of Israel, have I taken them unto me. For all the first-born among the children of Israel are mine, both man and beast: on the day that I smote all the first-born in the land of Egypt I sanctified them for myself. And I have taken the Levites instead of all the first-born among the children of Israel." — Numbers 8:16-18 (ASV)

Instead of such as open every womb, even instead of the firstborn of all the children of Israel, have I taken them unto me.

For all the firstborn of the children of Israel are mine, both man and beast: on the day that I smote every firstborn in the land of Egypt I sanctified them for myself. And I have taken the Levites for all the firstborn of the children of Israel.

God's people are the elect; they have escaped from death. In that day when the sword of the Lord was drawn, they were shielded by the blood of the lamb sprinkled on the lintel and on the two side posts; and, from that time on, because they have been preserved in this way, they belong to the Lord.