The concluding words are better expressed thus: “Mine shall they be, Mine, the Lord’s.” On the subject of the firstborn see the notes at Numbers 3:43-51.
Related Commentaries of Numbers 3:13
Numbers 3:13
13 Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.