1 Corinthians 1:1

We have here the apostle's preface to his whole epistle, in which we may take notice, I. Of the inscription, in which, according to the custom of writing letters then, the name of the person by whom it was written and the persons to whom it was written are both inserted. 1. It is an epistle from Paul, the apostle of the Gentiles, to the church of Corinth, which he himself had planted, though there were some among them that now questioned his apostleship (Co1 9:1, Co1 9:2), and vilified his person and ministry, Co2 10:10. The most faithful and useful ministers are not secure from this contempt. He begins with challenging this character: Paul, called to be an apostle of Jesus Christ, through the will of God. He had not taken this honour to himself, but had a divine commission for it. It was proper at any time, but necessary at this time, to assert his character, and magnify his office, when false teachers made a merit of running him down, and their giddy and deluded followers were so apt to set them up in competition with him. It was not pride in Paul, but faithfulness to his trust, in this juncture, to maintain his apostolical character and authority. And, to make this more fully appear, he joins Sosthenes with him in writing, who was a minister of a lower rank.

— Matthew Henry's Concise Commentary (public domain)