“For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith. For if they which are of the law be heirs, faith is made void, and the promise made of none effect: Because the law worketh wrath: for where no law is, there is no transgression. 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,” (Romans 4:13–16, KJV 1900)
When someone makes us a promise, we are at their mercy. We cannot make the promise happen by doing a thing. The fulfillment of the promise is solely in the hands of the one who made the promise. We must trust the one who made the promise, that he would fulfill it.
The promises of God to us are much different that those made by people. The difference is in the one making the promise. Man is inherently flawed in character, prone to be capricious and arbitrary, which means man is often unfaithful. On the contrary God is perfect, unflawed in character. What He says, He will do. It is completely outside of His infinitely holy and righteous essence to fail to keep a promise.
Abraham understood the character of God, and thus believed God. It is interesting to note, that Abraham believed God and it was counted to him as righteousness, over 500 years before the Mosaic law was given. Paul indicates that the promise did not come through the law. In fact, it would have been impossible for the promise to come through the law, since the law had not yet come. The promise came through the righteousness of faith.
Paul further states that if the promise came through the law, it would have voided faith and nullified the promise. Why? It is because by works of the law, no man could be justified. Thus, no one would be heir to the promise. Yet, God who is rich in mercy makes and fulfills His promises by grace through faith. We who believe in Christ and His atoning work for us, have been made heirs to the promise. We have a glorious future, not based upon our goodness or our doing any good works to sustain our salvation, but on God’s grace alone. While we are often unfaithful, He is always faithful.