Numbers 3:13

On the day that I smote all the firstborn. — The command given to Moses respecting the sanctification or separation of the firstborn, both of man and of beast, is recorded immediately after the account of the exodus and of the institution of the Passover (Exodus 13:1). It does not clearly appear, however, from the terms employed whether the sanctification or separation applied to the firstborn then in existence — which appears to be by far the more probable supposition — or whether, as some suppose, the command was simply prospective. The whole nation of Israel is described in Exodus 4:22 as the Lord’s firstborn son, and the firstborn sons appear to have been regarded in the light of representatives of the entire nation.

Mine shall they be: I am the Lord. — Or, They shall be (i.e., belong) to me, (even) to me, Jehovah. (Comp. Genesis 4:26 : “And to Seth, to him also.” Literally, And to Seth, even him.)