Galatians 3:15-18 (ESV)
To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified.  Now the promises were made to Abraham and to his offspring. It does not say, "And to offsprings," referring to many, but referring to one, "And to your offspring," who is Christ.  This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void.  For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise.
Paul makes a convincing argument. Once a covenant has been made, it cannot be added to or subtracted from. He makes the point that Abraham received a promise, not a law. The law came 430 years after Abraham. It cannot therefore subtract or add anything to the promise made to Abraham. Our inheritance, the forgiveness of sins and righteousness we receive in the New Testament, comes through a promise then, and not through the law. God gave the inheritance to Abraham by a promise. He extends this promise to us in Jesus Christ.