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,
gaining acquittal from guilt by His free unpurchased grace through the deliverance which is found in Christ Jesus.
Standing then acquitted as the result of faith, let us enjoy peace with God through our Lord Jesus Christ,
Notice therefore that those who possess faith are true sons of Abraham.
caused us, dead though we were through our offences, to live with Christ--it is by grace that you have been saved--
in order that having been declared righteous through His grace we might become heirs to the Life of the Ages in fulfilment of our hopes.
For when God gave the promise to Abraham, since He had no one greater to swear by, He swore by Himself,
2 Peter 1:10
For this reason, brethren, be all the more in earnest to make sure that God has called you and chosen you; for it is certain that so long as you practise these things, you will never stumble.
Before, not after. And he received circumcision as a sign, a mark attesting the reality of the faith-righteousness which was his while still uncircumcised, that he might be the forefather of all those who believe even though they are uncircumcised--in order that this righteousness might be placed to their credit;
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.