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Under the New Covenant, the promises of the Abrahamic Covenant are valid, and Abraham is our spiritual father, as it were. He is the model of the family, with whom God first made the covenant, and he obeyed God's voice (Genesis 26:5). He kept the commandments and the laws, and Abraham's children are going to do the same thing! Otherwise, they will not really be his children.
Paul is not doing away with the law! He is simply saying that the law cannot justify us. We see here, by God's own witness, that Abraham lived up to the terms of the covenant. Because he did, it was passed on to Isaac for him to do as his father had done.
The problem of transgressions in the Old Covenant was not resolved until the promised Seed, Christ, came. He lived perfectly, qualifying to be the payment for sin, and at the same time, He confirmed the promises that were made unto Abraham—and they were made absolutely and eternally binding. God then proposed the New Covenant that He had previously shown in prophecy (Jeremiah 31). God has presented it to all of mankind—not just to Abraham's physical descendants.
It is not circumcision that makes one a part of this covenant. Rather, it is circumcision of the heart! The sign is repentance and faith in the sacrifice of the promised Seed, Jesus Christ. The receipt of the Holy Spirit is the seal; it authenticates what has occurred. It completes the making of the New Covenant with the individuals whom God calls.
Nowhere does God say that the laws that define sin are done away. On the contrary, the One who made the New Covenant possible said that not one jot or tittle would pass from the law until all was fulfilled (Matthew 5:18).
God's moral and spiritual laws have been from eternity, and an agreement between Him and mere man is not going to do away with them. God Himself would have to pass from existence for that to occur. In addition, the loving intent of those laws as they apply to human relationships is still valid.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 27)