(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 13:1-2). 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.)

3:1-13 There was much work belonging to the priests' office, and there were now only Aaron and his two sons to do it; God appoints the Levites to attend them. Those whom God finds work for, he will find help for. The Levites were taken instead of the first-born. When He that made us, saves us, as the first-born of Israel were saved, we are laid under further obligations to serve him faithfully. God's right to us by redemption, confirms the right he has to us by creation.Because all the firstborn are mine,.... Not merely in a general way, as all creatures are his, but in a special manner as his own, and that for the following reason:

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; that is, sanctified or set them apart as his own special property, or ordered the people of Israel so to do, Exodus 13:2; for as when he destroyed the firstborn of the Egyptians, he saved the firstborn of Israel, he had a special claim upon them as his; and though it was in the night when he destroyed the firstborn of Egypt, yet it was the night which preceded that day, and was a part of that day, even the fifteenth of Nisan, when the instructions were given to sanctify all the firstborn; though, as Aben Ezra observes, "day" signifies "time", so that it was at or about the same time that the one and the other were done:

mine they shall be; this was declared when they were ordered to be sanctified to him, but now they were to be exchanged for the Levites:

I am the Lord; who have sovereign power to do as he would in claiming the firstborn, and then in exchanging them for the Levites, and appointing the Levites to minister to the priests, and serve in the tabernacle.