Romans 9 is sadly often abused to be used to support Calvin's doctrine of predestination which claims that God biases man's will to choose or not choose Him, therefore choosing who gets hell and who gets heaven as if history is a drama or puppet show God is putting on for Himself and the angels. Something like that.
Unfortunately, not only does the Bible not support this doctrine, but this doctrine is very destructive and antithetical to the spirit of Christ which is embodied throughout the Scripture. It is possibly even an "antichrist spirit" if I am permitted to say that.
So on to Romans 9.
Romans 9 is about not arguing with the God who wills who gets his grace/favor/blessing and who doesn't. So what does this mean? What does it have to do with?
First of all, it says that children of the flesh (Jews through Abraham's biological seed/genealogy) are not the children of God. This is Paul confronting the issue of Jew vs. Gentile. The Jews were angry the Gentiles were receiving God's grace by faith because they had worked hard to keep the law and believed they deserved grace and not the Gentiles. Paul then mentions God's choosing of Jacob over Isaac for certain plans to say that it's up to God how things work, not us. He chose to demonstrate favor on Jacob, not Isaac. It was His choosing, no one else's. It then says that he hardened Pharaoh's heart and used Him to demonstrate His glory; proceeding this statement, it says God hardens whom He will. Now it's important here to understand that Pharoah's free will was still involved. The answer to this lies in the Old Testament. In Exodus 7-10, Pharaoh first hardens his own heart, and then God finished the job. This demonstrates that God gives people a certain number of chances, and then stops and hardens their heart if they resist out of pride.
Paul goes on to talk about God's choosing and issues concerning Jews and Gentiles. This extensive passage summed up is Paul rebuking the Jews for not wanting God to give salvation to the Gentiles by faith when they've worked hard to keep the law. He's saying to them, "Who are you to question God and what he wills? So what if he wants to give them grace so easily. Who are you to be self-righteous about your works and contend with God?"
