Was the promise to Abraham that he would inherit the world given through the law or through faith?
The promise that Abraham would be heir of the world was not given through the law, nor through the promises made to Moses in the legal system. It came through the righteousness of faith. The law works wrath, but faith receives the promise.
Les explains that Paul, by inspiration, goes back to Abraham as the perfect example of being justified by faith and faith alone, plus nothing. Abraham was living in abject idolatry with no knowledge of the One True God, yet God saw in him the potential to believe. God reached down, spoke to him, and appeared to him in a physical form more than once. In verse 13, the promise that Abraham should be heir of the world was not through the law or the promises made to Moses in the legal system, but through the righteousness of faith. If those who are of the law be heirs, faith is made void and the promise made of none effect, because the law worketh wrath. The commandments can only bring wrath, not heaven. Les also notes he has never said grace means we do not keep the commandments; grace is not license.
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.