Romans chapter 9 is all about the sovereignty of God in His choosing of the nation of Israel. Paul is distraught over the fact that his people, the Israelites, are lost and their sins. They have rejected the Jesus as their Messiah, and are therefore without hope. However, God will keep His word to the nation of Israel, and they will be saved, and God will make a great nation from them, just as He promised Abraham that He would. Therefore, we can know that God will keep His word to believers as well, and that we can be assured of the salvation that we have in Christ.