The Church was one church in the book of Acts. St. Paul wrote to stay with what the Apostles taught, whether by word of mouth or letter. St. Paul stressed emphatically to not listen to anyone that taught anything different. Sounds like one church
quite true. It was one church made up of many independent churches made one in Christ. It was the church founded on Apostolic teaching and the traditions of the life of Jesus which eventually became the Gospels. The early church confirmed that those were the only acceptable traditions by rejecting all but the four Gospel in 2nd century AD. Those who belonged to that church were all who had come to our Lord Jesus Christ for salvation and rejected all tradition that was not according to the word of God as Christ had commanded.
There is no denomination or church today who can claim solely to represent that church. For today it is made up of all who have truly believed in Jesus Christ for salvation no matter what their denomination.