1 Corinthians 1:1

Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes, our brother,
1 Corinthians 1:1, Modern English Version (MEV)
 
The Apostle Paul makes it clear from the start that his apostleship, his ministry, is not something that he came up with on his own but is solely the result of God's call on his life, as is plainly evident when he met God on the road to Damascus. Paul maintained this throughout his ministry, and this is something that anyone called by God into His ministry must never forget.
 
Albert Barnes, in his Notes on the Bible, provides us with this additional background material:
 
Paul, called to be an apostle - See the notes at Rom_1:1.
 
Through the will of God - Not by human appointment, or authority, but in accordance with the will of God, and His command. That will was made known to him by the special revelation granted to him at his conversion, and call to the apostleship; Acts 9. Paul often refers to the fact that he had received a direct commission from God, and that he did not act on his own authority; compare Gal_1:11-12; 1Co_9:1-6; 2Co_11:22-33; 2Co_12:1-12. There was a special reason why he commenced this Epistle by referring to the fact that he was divinely called to the apostleship. It arose from the fact that his apostolic authority had been called in question by the false teachers at Corinth. That this was the case is apparent from the general strain of the Epistle, from some particular expressions 2Co_10:8-10; and from the fact that he is at so much pains throughout the two epistles to establish his divine commission.
 
And Sosthenes - Sosthenes is mentioned in Act_18:17, as “the chief ruler of the synagogue” at Corinth. He is there said to have been beaten by the Greeks before the judgment-seat of Gallio because he was a Jew, and because he had joined with the other Jews in arraigning Paul, and had thus produced disturbance in the city; see the note on this place. It is evident that at that time he was not a Christian. When he was converted, or why he left Corinth and was now with Paul at Ephesus, is unknown. Why Paul associated him with himself in writing this Epistle is not known. It is evident that Sosthenes was not an apostle, nor is there any reason to think that he was inspired. Some circumstances are known to have existed respecting Paul’s manner of writing to the churches, which may explain it:
 
(1) He was accustomed to employ an amanuensis (scribe) in writing his epistles, and the copyist frequently expressed his concurrence or approbation in what the apostle had indicted; see the note at Rom_16:22; compare Col_4:18. “The salutation by the hand of Paul,” 2Th_3:17; 1Co_16:21. It is possible that Sosthenes might have been employed by Paul for this purpose.
 
(2) Paul not unfrequently associated others with himself in writing his letters to the churches, himself claiming authority as an apostle; and the others expressing their concurrence; 2Co_1:1. Thus, in Gal_1:1, “all the brethren” which were with him, are mentioned as united with him in addressing the churches of Galatia; Php_1:1; Col_1:1; 1Th_1:1.
 
(3) Sosthenes was well known at Corinth. He had been the chief ruler of the synagogue there. His conversion would, therefore, excite a deep interest, and it is not improbable that he had been conspicuous as a preacher. All these circumstances would render it proper that Paul should associate him with himself in writing this letter. It would be bringing in the testimony of one well known as concurring with the views of the apostle, and tend much to conciliate those who were disaffected toward him.