"If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated: Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his."It's strange to me how this command of the Torah seems to condemn so much of what happened in Genesis. There are other areas where the behavior of the Patriarchs is in conflict with the Law given later, but this is the one area where God himself seems to be complicit, besides maybe incest where the change in attitude (we assume) was do to a change in biological circumstances.
We keep talking about a younger son getting treated as the firstborn being an ongoing theme of The Bible. But that theme doesn't apply to Jesus, who was the first born of both Mary and by adoption of Joseph, because all Joseph's other children were by Mary. And He is refereed to as the only Begotten Son of God.
The thing that occurred to me recently is, for the most part the inheritance this was done with isn't the natural Firstborn inheritance, the Double Portion, but rather special divine promises.
Let's start with Isaac and Ishmael. The land Abraham was promised went all the way to the Euphrates in Genesis 15:18. But the geography in Ezekiel 40-48 does not give the 12 tribes of Israel all of what Abraham was promised, rather their borders don't even go east of the Jordan (meaning it gives Israel no claim to any Land in modern Jordan or the Golan Heights). David ruled all the way to the Euphrates, but that was via other nations being tributaries, they still had their national sovereign identities outside the Tribal Allotments of Joshua.
Meanwhile the lands affiliated with Ishmael's sons include all of modern Jordan (Edom, Moab and Ammon were also in Jordan, but Edom I believe will not exist in the Messianic era, and the Nabateans conquered Edom, Moab and much of Ammon by NT times, and Isaiah 21 affiliated Dumah with Seir), the Golan heights, modern Syria south of Damascus (like the Iturians who were of Jether, and the Haggarenes/Haggarites), some northernmost parts of Saudi Arabia, and Iraq west of the Euphrates. Isaiah 60:7 confirmed Kedar and Nebaioth, the two oldest sons of Ishmael, will exist in the Messianic era. So it does seem that as far a geography goes Ishmael does still get twice what Isaac got.
What about applying the Firstborn inheritance to Nebaioth, Ishmael's Firstborn? During NT times there were two Nabatean Kingdoms, the one based in Petra ruled by the House of Aretas, and the Kingdom of Osroene ruled by the Abgars.
So I'm willing to agree with certain Muslim apologists that Genesis 17 and 21 do not at all disinherit Ishmael.
Now with Jacob/Esau and Pharez/Zerah in Genesis 38 we're dealing with twins, where who is the real first born is legitimately ambiguous. Still all Pharez got over Zerah is the Royal Line and other leaders. And Esau sold his inheritance, while Jacob suffered repercussions for stealing the Blessing.
Genesis 10 says Japheth was the firstborn of Noah, and God said in Genesis 9 he would enlarge Japheth.
In Genesis 48 the younger son of Joseph, Ephraim, is given a special blessing and promise. But in the land allotment Manasseh still got twice the land Ephraim did, even based on the larger interpretation of what Ephraim got (giving Ephraim both Shechem and Samaria), Manasseh had huge portions on each side of the Jordan river. So in fact Manasseh's firstborn inheritance was in no way taken away by this. In the Song of Deborah, Manasseh is treated as two tribes, Machir and Gilead.
So it's really only Jacob who would have been in direct violation of this command given in Deuteronomy, by giving the Double Portion to Joseph (which is why Joseph gets two tribes). And while it can seem unfair to judge him by a law not written yet, the consequences of his choice are very apparent, God respected it, but it's still not painted as a good idea.
The same applies to looking at how this plays out with David.
David was the youngest son of Jesse and was chosen to be King over his seven older brothers. I would argue that was irrelevant to who inherited Jesse's land, if anything David was effectively removed from that inheritance. I like to speculate that the Shepherds in Luke 2 were descendants of David's older brothers, being called to witness the birth of their distant Cousin. Some scholars already believe their fields were the same land Boaz redeemed for Naomi.
And then there's David's own sons. Solomon was not even the oldest by Bathsheba who was far from David's first wife. But still he inherited the Crown. The King arguably didn't have a normal land inheritance.
David's actual firstborn was Amnon who predeceased him. And either way you could argue lost any potential inheritance by the Sin he committed.
That leaves Daniel Chileab as the oldest surviving son. He was the son of Abigail the Widow of Nabal, David gained Nabal's land when he married Abigail, so perhaps it's a safe guess that Daniel Chileab inherited Nabal's land?
Absolam and Adonijah effectively forfeited their inheritances by becoming usurpers, not unlike Reuben.
Maybe the remaining sons born in Hebron inherited Hebron in some fashion?
And perhaps the older sons of Bathsheba inherited land that once belonged to Urias?