Charles Ellicott Commentary Exodus 13:2

"Sanctify unto me all the first-born, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine." — Exodus 13:2 (ASV)

Sanctify unto me all the firstborn. —It was a reasonable demand that the existing firstborn of Israel, spared by God when the Egyptian firstborn were destroyed, should be regarded from then on as His, and set apart for His service. The extension of the demand to existing animals was also reasonable, since they too had been spared. God’s further requirement, that from now on all the future firstborn should also be His, was intended to perpetuate the memory of the recent deliverance and to help fix it in the mind of the nation.

The substitution of a redemption in the case of unclean animals was necessary because they could not be sacrificed; and the redemption of the firstborn sons naturally followed when the Levitical priesthood was established, and their services were no longer necessary. (Numbers 18:16.) The Jews still observe the ordinance, as far as the children are concerned, and redeem the son who has “opened the womb” on the thirtieth day after the birth.