The firstborn inherited everything from the father. Christ is the firstborn son of God who, as a son of man, inherited all of the promises to Abraham, Isaac and Jacob. Believers in him are called the assemply of the firstborn, ie Christ. He inherited all things and died, so now those born of his spirit become heirs of the promises through him who lives forever.
As an example, Ehpraim was the firstborn who inherited the promises to Abraham, Isaac and Jacob. The other tribes did not inherit the promises, but they enjoyed allotments of Ephraim's inheritance through their association with him. That is why Israel said this when the kingdom split apart:
So when all Israel saw that the king hearkened not unto them, the people answered the king, saying, What portion have we in David? neither [have we] inheritance in the son of Jesse: to your tents, O Israel: now see to thine own house, David. So Israel departed unto their tents. 1 Kings 12:16
And why Ezekiel described Israel as Ephraim and his associates
Say unto them, Thus saith the Lord GOD; Behold, I will take the stick of Joseph, which [is] in the hand of Ephraim, and the tribes of Israel his associates, and will put them with him, [even] with the stick of Judah, and make them one stick, and they shall be one in mine hand. Ezekiel 37:19
So all of this points to the fact that we receive an allotment of the inheritance through our association through faith with the firstborn son of God who inherited everything. This is what I call the genealogy of faith.