Treasury of Scripture Knowledge
Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
and are justified freely by his grace through the redemption that is in Christ Jesus,
Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ,
Therefore, be sure that it is those who are of faith who are sons of Abraham.
even when we were dead in our transgressions, made us alive together with Christ (by grace you have been saved),
so that we might be justified by his grace and become heirs according to the hope of eternal life.
For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself,
2 Peter 1:10
Therefore, brethren, be all the more eager to make your calling and election sure. For if you do these things, you will never fall;
And he received the sign of circumcision, a seal of the righteousness which he had by faith while he was still uncircumcised, that he might be the father of all who believe but have not been circumcised, in order that righteousness might be reckoned to them.
In other words, it is not the children of the flesh who are the children of God, but the children of the promise are reckoned as descendants.