Paul, called to be an apostle of Christ Jesus by the will of God, and our brother Sosthenes, to the church of God that is in Corinth, to those who are sanctified in Christ Jesus, called to be saints, together with all those who in every place call on the name of our Lord Jesus Christ, both their Lord theirs and ours: Grace to you and peace from God our Father and the Lord Jesus Christ. (1 Corinthians: 1-3)

The co-author - or at least co-sponsor - of the letter is Sosthenes. In Acts of the Apostles 18: 12-17 a Sosthenes is the head of the Corinthian synagogue. We cannot be sure that this is the same Sosthenes.

During the first century many non-Jews associated themselves with synagogue worship but did not fully convert. It was among these gentile "God-fearers" that Paul most often focused his work.

For the head of a synagogue to join Paul would be unusual. In Acts that Sosthenes is beaten by other Jews who have brought charges against Paul to the Roman proconsul. So it would seem that in Corinth Paul had converts among both circumcised and uncircumcised.