Sosthenes was the synagogue ruler in Corinth. At that time, the Jews accused Paul, but when Governor Gallio released Paul, they beat Sosthenes.
Acts 18:12While Gallio was proconsul of Achaia, the Jews made a united attack on Paul and brought him to the tribunal,Acts 18:13saying, “This man is persuading people to worship God contrary to the law.”Acts 18:14But when Paul was about to open his mouth, Gallio said to the Jews, “If it were a matter of wrongdoing or vicious crime, O Jews, I would have reason to accept your complaint.Acts 18:15But if it is a matter of questions about words and names and your own law, see to it yourselves. I refuse to be a judge of these things.”Acts 18:16And he drove them from the tribunal.Acts 18:17Then all the people seized Sosthenes, the ruler of the synagogue, and beat him in front of the tribunal. But Gallio paid no attention to any of this.
Up until this point, there is no record of Sosthenes hearing the gospel and converting. However, given that his name appears as a sender in 1 Corinthians, it seems he converted around this time.
1 Corinthians 1:1Paul, called by the will of God to be an apostle of Christ Jesus, and our brother Sosthenes,