Paul was deep into his discussion of Abraham's righteousness by faith that was available to both Jews and Gentiles. He continued.
Romans 4:12-16 (KJV)
And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
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,
Paul summarized his discussion to this point in verse 12. It was by faith that Abraham received God's righteousness. He received it before he was circumcised, therefore he was the father not only of the Jews, but also the Gentiles who walked by faith in God.
Then in verse 13 Paul began to speak about the promise that God had given to Abraham that he and his descendants would become the heirs of the world (This likely refers to Abraham becoming the father of many nations or the nations being blessed by him.). This promise didn't come by the observance of Torah, but by the righteousness Abraham received through faith.
If the promise had come by being Jewish, then faith was made void and the promise was made of no effect (verse 14). Paul was saying that if only Jews could participate and the receiving of the promise was conditional upon observance of the Torah, faith didn't matter. Also, since Paul had already established that no one had perfectly observed the Torah, the promise would never come about.
In verse 15 Paul stated that the Torah works wrath. In other words, the Torah is a book of instructions. When the instructions are transgressed there is punishment. This is what the Jews deserved. However, the Gentiles were in an even worse position. Where the Torah is not taught, legally there is no transgression. Yet, the Gentiles, as Paul had been arguing, were not without culpability. They did what was right in their own eyes and incurred the wrath of God as well.
If the promise came by being Jewish or by observing Torah, there was no hope. But thankfully, the promise comes by faith (verse 16). It is a result of the grace of God rather than through any act or native inheritance of the recipient. It is therefore available to all the "descendants" of Abraham, to Jews and Gentiles who have faith in Yeshua.
Next will come part 4!