Why do Gentile believers get included in the promises made to Abraham?
Gentile believers are included because Abraham's promises extended beyond the nation of Israel. Abraham was saved by faith plus nothing, so all who believe the Gospel become one in Christ and members of His Body, making them spiritual heirs and children of Abraham, not through earthly descent.
Les explains that as soon as we become a believer of the Gospel, we are one in Christ and in the Body, and we then become heirs of the promises made to Abraham. This is because Abraham's promises went way beyond the Nation of Israel, so Gentile believers come in there. Abraham himself was saved by faith plus nothing: he did not bring sacrifices for salvation, was not circumcised before salvation, and was not under the Law; he simply believed God. In our age of grace, we are saved the same way, faith plus nothing. That ties us to Abraham not through earthly progeny but only in the spiritual realm, where we become the children of Abraham.