Romans 4:13
For the promise that he should inherit the world did not come to Abraham or his descendants through Law, but through the righteousness due to faith.
Galatians 3:29
And, since you belong to Christ, it follows that you are Abraham's offspring and, under the promise, sharers in the inheritance.
Romans 9:8
This means that it is not the children born in the course of nature who are God's Children, but it is the children born in fulfillment of the Promise who are to be regarded as Abraham's descendants.
Galatians 3:16-18
Now it was to Abraham that the promises were made, 'and to his offspring.' It was not said 'to his offsprings,' as if many persons were meant, but the words were 'to thy offspring,' showing that one person was meant--and that was Christ.