Charles Spurgeon Commentary 1 Corinthians 1:1-2

"Paul, called [to be] an apostle of Jesus Christ through the will of God, and Sosthenes our brother, unto the church of God which is at Corinth, [even] them that are sanctified in Christ Jesus, called [to be] saints, with all that call upon the name of our Lord Jesus Christ in every place, their [Lord] and ours:" — 1 Corinthians 1:1-2 (ASV)

Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother, unto the church of God which is at Corinth, (1 Corinthians 1:1–2)

Note the humility of Paul in associating with himself an almost unknown brother, Sosthenes. Although the letter is written by Paul alone, yet, as if he did not care to stand in isolation even for a moment, he associates Sosthenes with himself in the salutation: Unto the church of God which is at Corinth,.