Paul probably wrote at least four letters to the church at Corinth. Two of them are included in the New Testament; there are no known copies of the others. First Corinthians 5:9-11 indicates that Paul had written a letter to the Corinthians previous to First Corinthians.
Paul arrived by himself in Corinth on the Second Missionary Journey. He made acquaintances with Priscilla and Aquila and began working with them; Silas and Timothy joined them later. Together, they established the church in Corinth. Paul stayed in Corinth for about a year and a half. There is no doubt he had strong relationships with the people and knew them individually.
First Corinthianswas written about three or four years after Paul left Corinth, toward the end of the Second Missionary Journey. The letter is addressed from Paul and Sosthenes, a former resident of Corinth who was previously the leader of its synagogue.
By the time of the letter, the church at Corinth had become a disaster, forcing Paul to address a long list of problems. The church was divided over who they counted as their leader; rather than following Christ, some of them wanted to follow Paul, Apollos, or Peter. Paul had to convince them that Christ is the head of the church, and all others are followers who use their gifts to serve the Lord.
Once again, Paul found it necessary to prove his apostleship and show why he had the authority to teach and be followed. Apparently, the Corinthians were either being influenced by false teachers or were being so prideful they simply did whatever they wanted.
The Corinthians chose to overlook a case of shameful incest among their members. A man was sleeping with his stepmother, and that church did not mourn about it. In fact, the church members were proud of it. Paul insisted that they put the man out of their fellowship. Paul made it clear that they must not even associate with a church member who is sexually immoral, greedy, an idol worshiper, slanderer, or drunkard. As evidenced by this letter, the Corinthians had a lot to learn about living as Christians.