'Firstborn Son' in the Bible

And they were as follows: The descendants of Reuben, the firstborn son of Israel: According to the records of their clans and families, all the males twenty years old or older who could serve in the army were listed by name individually.

"Separate the descendants of Levi in place of all the firstborn sons of Israel and the livestock of the descendants of Levi in place of their livestock. The descendants of Levi belong to me, since I am the LORD.

For they are entirely given to me from among the Israelites. I have taken them for myself instead of all who open the womb, the firstborn sons of all the Israelites.

So I have taken the Levites instead of all the firstborn sons among the Israelites.

The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, and the firstborn males of unclean animals you must redeem.