John Calvin Commentary Numbers 3:12

"And I, behold, I have taken the Levites from among the children of Israel instead of all the first-born that openeth the womb among the children of Israel; and the Levites shall be mine:" — Numbers 3:12 (ASV)

And I, behold, I have taken the Levites. A little further on we will see more clearly why God claims one tribe for Himself. He now only shows that the Levites rightfully belong to Him, because by special privilege the first-born of the people were preserved in the destruction of the Egyptians. God, therefore, declares that those whose lives were thus graciously spared were purchased for Himself.

Since, then, He had the free option of devoting to Himself the first-born of every tribe, He was no less at liberty to take ⁴²⁵ only the twelfth part of the people from one tribe. Thus He removes all basis for complaint, since it would have been intolerable ingratitude to withdraw from His control those whom He had miraculously redeemed. Therefore, He says that they did not perish in the general slaughter so that He might subject them to Himself.

⁴²⁵ “Moins que la douzieme partie;” less than the twelfth part. — Fr.