I went to Barcelona in October 2000 and we went out partying, went to a place that was a cathedral(?) located up on a hill and they had a club in the building that once had been a cathedral, several dancefloors and you could even go up to the roof of the place where there was a bar and chill out. What was the name of this place in Barcelona? Does it still exist?
It is likely you are thinking of El club La Terrrazza (note the 3 'r's). You go up a hill through a quaint village with twisty sort of small streets to get there.
It's a converted villa rather than a cathedral, and the 'roof' you wrote about is most likely their balcony. The club opened in 1995 so meets your dating criteria of October 2000. Also, it has added new 'wings' and dance floors since your last visit.