Paul writes his letter and begins it by noting that it is sent from he and his brother Sosthenes. But who is this Sosthenes?
While we cannot be sure who this refers to, there is a Sosthenes mentioned in chapter 18 of the book of Acts.
12 But when Gallio was proconsul of Achaia, the Jews made a united attack on Paul and brought him before the tribunal, 13 saying, "This man is persuading people to worship God contrary to the law." 14 But 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. 15 But since 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." 16 And he drove them from the tribunal. 17 And they all seized Sosthenes, the ruler of the synagogue, and beat him in front of the tribunal. But Gallio paid no attention to any of this.
This Sosthenes was a Jewish leader who, when his plans to attack Paul ended in an embarrassing rejection by the Roman ruler Gallio, was beaten and rejected by his own people. It is not far fetched to think that this man that was beaten and isolated may have been one that Paul himself would have approached, showing compassion. In so doing maybe this onetime enemy of Paul became a friend of the church and a brother. Paul probably would have had a special sympathy for Jewish leaders persecuting the church, for that was Paul’s own history back when he was Saul.