Paul is writing to the Corinthians from Ephesus during the fall of 55 AD or the spring of 56 AD. Timothy and a delegation have been sent to Corinth, but Apollos was unwilling to go back with the delegation even though Paul urged him to go and the Corinthians were asking for him (1 Corinthians 16:10-12). The delegation that is traveling with Timothy to Corinth are leaders of the Corinthian
church that had traveled to Ephesus to meet with Paul and are identified in 1 Corinthians 16:17 (Stephanas, Fortunatus and Achaicus) and are also mentioned in 1 Corinthians 1:11. The delegation
is returning to their church in Corinth with Timothy and a letter from Paul that we
call First Corinthians.
In this letter Paul told the Corinthians that his plans were to stay in Ephesus until Pentecost
(between May 16–June 13) in the spring of 56 AD (which would be 50 days after the Jewish Feast
of Passover held between March 26-April 24). Paul stayed in Ephesus a few more months before he left for Corinth. Once Paul is in Corinth, he will try to leave for Jerusalem before Pentecost in the spring of 57 AD at the end of his third missionary journey according to Acts 20:16 .
The reason Paul sends Timothy to Corinth with the delegation from Corinth is “because a great
door for effective work has opened” for him in Ephesus and the surrounding cities of Asia. It would appear that it was during this time that the churches of Colossae, Smyrna, Laodicea, Philadelphia, etc. were started. The effective work of Paul and the Ephesians at this time is described in
"He took the disciples with him and had discussions daily in the lecture hall of Tyrannus. This went on for two years, so that all the Jews and Greeks who lived in the province of Asia heard the word of the Lord.” – Acts 19:9-10
Not only was there “a great door for effective work” in Ephesus, Paul also says, “there are many
who oppose me.” It seems Paul was anticipating the conflict stirred up by Demetrius that would
result in the riot that drove Paul out of Ephesus a few months after the letter of First Corinthians
was sent, because this opposition eventually resulted in the riot of Acts 19:23-41. The
“effective work” of the ministry altered the cultural and spiritual climate of Ephesus to the point
that the economy created by the sale of the silver idols produced by Demetrius and the other craftsmen was seriously impeded. The presentation of Bible teaching and the apostolic doctrine
on a daily basis to the believers in Ephesus changed their lives, their culture and, then, reached
into the surrounding cities of Asia.