In the tradition of letter writing in the Roman world during the first century, both the senders, Paul and Sosthenes and the recipients, the church at Corinth, are identified in the opening of the epistle of 1 Corinthians.
Paul was an apostle who had been “sent” by God primarily to reach the Gentiles with the gospel of Jesus Christ. Sosthenes, Paul identified as a brother in Christ. He was probably the same Sosthenes who is identified in Acts 18:17 as the ruler of the synagogue in ancient Corinth.
The church in Corinth was the church of God! The church was/is God’s for it was purchased by the blood of Christ. Those who make up the church are sanctified, set apart to God or holy. Every member of Christ’s body, the church, in every time and place, are called by God to be saints or holy ones. That Paul calls the Corinthians holy is amazing given the problems they had and the sins they committed. But a saint is not a saint because of his lifestyle, but because of God’s call!
We, as Christians, have been set apart by God. We are holy in His eyes, but as Peter wrote, “But just as he who called you is holy, so be holy in all you do; 16 for it is written: “Be holy, because I am holy.” (1 Peter 1:15-16)