ISB ~ 1 Corinthians 1 v1

called by the will of God:Paul’s evangelical mission was established on God’s initiative, not his own (Acts 9:1-16; Gal 1:12). He asserts his apostolic authority from the outset of the letter because some of the Corinthians either doubted or denied it (9:1-2; 2 Cor 10-12).

Sosthenes:Possibly the ruler of the Corinthian synagogue named in Acts 18:17. If so, he must have embraced the gospel during Paul’s initial stay in the city.