Acts 2. They were saved, then baptized, then added to the church.

Acts 2:41 Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.

Acts 2:46-47 And they, continuing daily with one accord in the temple, and breaking bread from house to house, did eat their meat with gladness and singleness of heart, Praising God, and having favour with all the people. And the Lord added to the church daily such as should be saved.

You may differ on your understanding of HOW someone gets added to the church - but it is obvious it was saved, baptized believers that were members - they did not automatically become members of a local church just upon salvation.

Then in 1 Corinthians 5, it tells us to chastise and put unrepentant members of the church out - can't do that unless there is a clear distinction between those believers that are in a church and those that are out.