Galatians 3:6-9
Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore that they which are of faith, the same are the children of Abraham. And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. So then they which be of faith are blessed with faithful Abraham.
Anyone who believes onto Christ is considered a child of Abraham. This is important because that the true promise to Abraham was his children will receive salvation. Our faith in Christ is what connects us to Abraham. We are his children.