Were there Jewish believers in the Corinthian church?
Yes, Jewish believers were part of the Corinthian church. The context indicates that the called include both Jew and Greek, and it concludes this is another reason to believe there were Jews also among the believers in the Corinthian church.
Les teaches that the called of Scripture—the elect, the saints, and the believers—are all one and the same. In the context, Paul addresses believers whether Jew or Greek, and Les says this gives another reason to think there were Jews in the Corinthian church. He also notes Jews and Greeks together, and later contrasts the wisdom of God with the wisdom of men, stressing that only God's wisdom makes a person usable.
But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.