What did a person have to believe to become a believer under Judaism?
To become a believer under Judaism, a person had to believe that Jesus was the Christ, that He was who He said He was. The Pharisees who became believers held this belief, and it was the required belief for becoming a believer under Judaism.
Les explains that the Pharisees mentioned in Acts 15 were not Christ-rejecting Pharisees but believers—Pharisees who now believed that Jesus was the Christ. That belief, that Jesus was who He said He was, was what a person had to believe to become a believer under Judaism. These believing Pharisees then demanded that Gentile believers be circumcised and keep the law of Moses, but Paul resisted, showing that Gentiles were now under his gospel of grace, not the law.