We're the only ones here!

I heard this joke when listening to some of Chuck Smith's (of Cavalry Chapel) Bible Study:

A man died and went to heaven, and Peter the Apostle was showing him around. Around the heavenly city Christians were singing praises, hymns and all sorts of songs. There were Lutherans, Presbyterians, Baptists, Calvinists, Arminians, Quakers, Mennonites, all sorts of groups all singing together for the Glory of the Lord.

The man stopped at an enclosed building, with walls all around, yet he could still hear Hymns, praises and singing in general inside the building. He turned and asked Peter "What is this?"

Peter answered "Oh. That's the Church of Christ. They think they're the only ones here"