Numbers 3:12

Overview - Numbers 3
The sons of Aaron.
The Levites are given to the priests instead of the first-born;
14 are numbered by their families.
21 The families, number, and charge of the Gershonites;
27 of the Kohathites;
33 of the Merarites.
38 The place and charge of Moses and Aaron.
40 The first-born are freed by the Levites.
44 The overplus are redeemed.
Treasury of Scripture Knowledge

Numbers 3:12  (King James Version)
And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;

When God miraculously destroyed all the first-born of the Egyptians, ( Exodus 12:29, ,)he spared those of the Israelites; and, in commemoration of that event, he was pleased to appoint that all the first-born males "should be set apart unto himself
" ( Exodus 13:12-16 .) God is here pleased to relinquish this claim, and to appoint the whole tribe of Levi to attend his immediate service in their stead.
Numbers 3:41 Numbers 3:45 ; Numbers 8:16 Numbers 8:18 18:6