Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother, Paul, called to be an apostle - Bishop Pearce contends that a comma should be placed after κλητος, called, which should not be joined to αποστολος, apostle: the first signifies being called to, the other sent from. He reads it, therefore, Paul the called; the apostle of Jesus Christ. The word κλητος, called, may be here used, as in some other places, for constituted. For this, and the meaning of the word apostle, see the note on Romans 1:1.
As the apostle had many irregularities to reprehend in the Corinthian Church, it was necessary that he should be explicit in stating his authority. He was called - invited to the Gospel feast; had partaken of it, and, by the grace he received, was qualified to proclaim salvation to others: Jesus Christ therefore made him an apostle, that is, gave him a Divine commission to preach the Gospel to the Gentiles.
Through the will of God - By a particular appointment from God alone; for, being an extraordinary messenger, he derived no part of his authority from man.
Sosthenes our brother - Probably the same person mentioned Acts 18:17 (note).