Or a relationship? Just say you have 20 good Christian friends: How do you institutionalize that set of friendships so that those 20 people can identify themselves as an institutional church? Can spiritual fellowship and love be institutionalized? Can friendship?
Why do some people have Christian friends, and yet they don’t consider their fellowship with them as being church? Then, what is the “magical ingredient” which transforms fellowship into “church”?
Last Days of the Age 
Brother Ian made some great comments to add to these excellent questions. Stop over and check it out.