Romans 4:13
Again, the promise that he should inherit the world did not come to Abraham or his posterity conditioned by Law, but by faith-righteousness.
Galatians 3:29
And if you belong to Christ, then you are indeed true descendants of Abraham, and are heirs in fulfilment of the promise.
Romans 9:8
In other words, it is not the children by natural descent who count as God's children, but the children made such by the promise are regarded as Abraham's posterity.
Galatians 3:16-18
(Now the promises were given to Abraham and to his seed. God did not say 'and to seeds,' as if speaking of many, but 'and to your seed,' since He spoke of only one--and this is Christ.)