The reason why Paul wants to stay in Ephesus until Pentecost is so that he can do the work in that city which the Lord seems to be calling him to do. Interestingly, Paul mentions effective work
and many adversaries
in one breath. This is a reminder that, wherever Christ is building his church, Satan will try his best to destroy it.
In Acts 19 we are given glimpses of Paul’s ministry in Ephesus. After having reasoned with the Jews for three months, he withdrew from the synagogues and took the disciples with him, reasoning daily in the hall of Tyrannus.
He did this for two years, until all the residents of Asia heard the word of the Lord, both Jews and Greeks
(Acts 19:9–10). In the same chapter we read of two groups that became Paul’s fierce adversaries: the unbelieving Jews and the craftsmen who made and sold idols.
9 for a wide door for effective work has opened to me, and there are many adversaries.