Paul showed the Greek and Roman world a new approach to religion. He showed them a religion that was based on grace, faith and unmerited favor. It was by God's grace that men were saved through the shed blood of Jesus Christ (Romans 3:24).
Before Jesus Christ, religion was based on works. Religion was based on how hard a man worked at his salvation. It was based on merits, not favor. Many religious people were justified by their own goodness and ability to work for their faith.
Paul's doctrine was new. He was preaching about a religion that was based on grace and faith through the sacrifice that Jesus Christ did on the cross. Now, there was a way that man could be saved through his faith rather than his works. This new religion was based on a man's belief in Jesus Christ and that he was crucified, buried and resurrected on the third day (Romans 10: 9). Faith alone could save a man. Through no goodness on a the part of oneself could one be saved. This new doctrine astounded the religious leaders. They were extremely upset because they believed in their own goodness through their own works.
Paul understood their thinking for he too had been a Pharisee, a religious person who believed in the works of man as his own personal salvation. For Paul, a former Pharisee, to be preaching this new doctrine, it was astounding. To believe that a man could be saved by his mere belief in a crucified, buried, and resurrected Jesus Christ was astonishing teaching.
Paul preached that anyone could be saved. This new message infuriated the religious sect known as the Jewish Pharisees. Now, Paul is preaching to the non-Jewish, claiming that they can be saved if they have faith in Jesus Christ. Paul preached that a man's simple faith could save him (Romans 10: 9-10).