“So then it depends not on human will or exertion, but on God, who has mercy” (Romans 9.16 ESV).
Paul had such a deep understanding of faith that he could share the Gospel with almost anyone and make a case for Jesus that was hard to dismiss. Although he is many times called the Apostle to the Gentiles, he was also excellent with presenting the Gospel to the Jewish people and to people who understood Jewish law. Paul himself was of the strict sect of the Pharisees, so he understood how their thought systems worked. One of the main difficulties the Jewish people had with accepting a Savior was turning from trying to achieve their own righteousness (which none of them could ever accomplish) to freely receiving righteousness by faith through Jesus Christ. The other difficulty they had was realizing that they weren’t children of God just because they were descendants of Abraham. Both of these paths to God were based on human effort, and Paul wanted them to see that our righteousness (right standing with God) was now based solely on God’s effort via the death and resurrection of Jesus Christ. As humans, we could never earn or achieve our way to heaven. It doesn’t matter what family we were born to or how good we think we are. We cannot gain right standing with God on our own. God knew this, so He implemented a plan to bring us to the end of ourselves historically as a people and individually as a child of God, so we would see our need for a Savior. Only when we see our need, will we reach outside of our human efforts and admit that we need a Savior.
“For all have sinned, and come short of the glory of God” (Romans 3.23 KJV).