Genesis 17:19 - Then God said "Sarah your wife shall bear you a son, and you shall call his name Isaac; I will establish My covenant with him for an everlasting covenant, and with his descendants after him.
Romans 9:6-8 - But it is not that the word of God has taken no effect. For they are not Israel who are of Israel, nor are they all children because they are tge seed of Abraham; but, in Isaac your seed shall be called. That is, those who are the children of tge flesh, these are not the children of God; but the children of promise are counted as the seed.
The covenant which is promised to Isaac's seed is the promise of eternal life. This covenant was fulfilled by the birth, crucifixion, resurrection, and ascension of Jesus Christ. These verses in Romans clearly explains that salvation does not come from birthright or lineage. Our salvation only comes from our belief in Jesus Christ (see John 3:16-18).