The church at Ephesus started with Paul discovering some disciples on his third missionary tour. They only knew the baptism of John and had heard nothing about the Holy Spirit. Evidences show that they might have been students of Apollos (Acts 18:25, 19:1-3).
Ephesus was made up of both Jews and Gentiles. Hence, it is very possible that both groups were represented in Paul’s congregation (Acts 19:10 & 17, 20:21). This reveals why Paul addressed some issues in the letter, such as unity; saying that Jews and Gentiles have been made one in Christ (Ephesians 2:14-15).
Paul taught them more accurately about the faith, baptized them and got them filled with the Holy Ghost. They spoke in tongues and prophesied (Acts 19:4-6).
He taught them for 3 years (Acts 20:31) before handing them over to his co-workers such as Timothy and later Tychicus (Ephesian 6:21)
This church obviously deviated from the instructions Paul gave them, for Christ said this about them “You have forsaken your first love” (Rev 2:4). Paul himself prophesied this while he was giving his farewell speech to the elders of the church in Ephesus at Miletus (Acts 20:29-30).