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Christians do not consider Isaac to be the firstborn son of Jacob.
Esau and Jacob were twins born to Isaac. Esau was the eldest.
Genesis 25:21 Now Isaac pleaded with the LORD for his wife, because she was barren; and the LORD granted his plea, and Rebekah his wife conceived... 24 So when her days were fulfilled for her to give birth, indeed there were twins in her womb. 25 And the first came out red. He was like a hairy garment all over; so they called his name Esau. 26 Afterward his brother came out, and his hand took hold of Esau’s heel; so his name was called Jacob. Isaac was sixty years old when she bore them.
Esau sold his birthright to Jacob.
Genesis 25:29 Now Jacob cooked a stew; and Esau came in from the field, and he was weary. 30 And Esau said to Jacob, "Please feed me with that same red stew, for I am weary." Therefore his name was called Edom. 31 But Jacob said, "Sell me your birthright as of this day." 32 And Esau said, "Look, I am about to die; so what is this birthright to me?" 34 And Jacob gave Esau bread and stew of lentils; then he ate and drank, arose, and went his way. Thus Esau despised his birthright.
Jacob stole Esau’s blessing.
Genesis 27:35 But he (Isaac) said, "Your brother came with deceit and has taken away your blessing." 36 And Esau said, "Is he not rightly named Jacob? For he has supplanted me these two times. He took away my birthright, and now look, he has taken away my blessing!"
The promises to Abraham and Isaac come through Jacob and not Esau.
Genesis 28:10 Now Jacob went out from Beersheba and went toward Haran. 11 So he came to a certain place and stayed there all night, because the sun had set. And he took one of the stones of that place and put it at his head, and he lay down in that place to sleep. 12 Then he dreamed, and behold, a ladder was set up on the earth, and its top reached to heaven; and there the angels of God were ascending and descending on it. 13 And behold, the LORD stood above it and said: "I am the LORD God of Abraham your father and the God of Isaac; the land on which you lie I will give to you and your descendants. 14 "Also your descendants shall be as the dust of the earth; you shall spread abroad to the west and the east, to the north and the south; and in you and in your seed all the families of the earth shall be blessed.
The seed referred to is Jesus Christ born of the tribe of Judah, the 4th son of Jacob.
Galatians 3:16 Now to Abraham and his Seed were the promises made. He does not say, "And to seeds," as of many, but as of one, "And to your Seed," who is Christ.
Galatians 3:29 And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise.
The promise made to Abraham and his seed is Eternal life and possession of the earth by resurrection from the dead and this is the true Christian hope.
Deuteronomy 34:4 And the LORD said unto him (Moses), This is the land which I sware unto Abraham, unto Isaac, and unto Jacob, saying, I will give it unto thy seed: I have caused thee to see it with thine eyes, but thou shalt not go over thither.
Isaiah 2:1-4 Now it shall come to pass in the latter days That the mountain of the LORD’S house Shall be established on the top of the mountains, And shall be exalted above the hills; And all nations shall flow to it. 3 Many people shall come and say, "Come, and let us go up to the mountain of the LORD, To the house of the God of Jacob; He will teach us His ways, And we shall walk in His paths." For out of Zion shall go forth the law, And the word of the LORD from Jerusalem. 4 He shall judge between the nations, And rebuke many people; They shall beat their swords into plowshares, And their spears into pruning hooks; Nation shall not lift up sword against nation, Neither shall they learn war anymore.
This will happen when Christ returns from heaven.
I hope this helps.