The letter (epistle) of First Corinthians was written by Paul, the apostle, to the church of God in the city of Corinth. These people had already received the gift of holy spirit from God by means of the Lord Jesus Christ because they had already been taught and they believed God's Word. The Corinthian church consisted of holy-people (Christians) who were of a Judean/Israeli background and also those who were of a Gentile background. They had written to Paul asking for his help regarding certain topics which had arisen within their church. Also, he had heard from other holy-people regarding certain events that were taking place within the Corinthian church.
Paul wrote this letter by revelation to remind the holy-people and to teach them more of that-which God has given mankind to believe regarding Himself and His son, the Lord Jesus Christ. He wrote this letter at Ephesus in the spring of 57 AD (refer to Acts chapter 19 and I Corinthians 16:8).
Corinth was the chief city of the Roman province of Achaia, which was in the southern part of modern-day Greece. It had a harbor on either side of it, and so it was a large commercial city, which resulted in the fact that some of its citizens were very wealthy and they lived wildly and extravagantly.
When we read the contents of Paul's letter to the Corinthian church, we can see how adversely the people who did not believe God's Word in that city were affecting the behavior of some of the holy-people.