Brotherofwill said:
If you want to see some crazy religion inspired architecture then you should check out Gaudi's "Church of the Holy Family" in Barcelona. I visited some years ago, totally blew me away
Ah, La Sagrada Familia, I was going to bring that up. I was there over the summer, absolutely amazing. Speaking of Gaudi, have you been to Parc Guell? [http://en.wikipedia.org/wiki/Park_g%C3%BCell] Not religious architecture, but still...amazing.
Also, while in France, I went to a church designed by Matisse, seen here [http://en.wikipedia.org/wiki/Chapelle_du_Rosaire_de_Vence], near Vence. I personally didn't like it, as it felt oppressive inside (small + stark white walls with simple drawings in black + very simple colors where there are any = claustrophobia), but it's still quite interesting.
Last one, a Basilica [http://en.wikipedia.org/wiki/Basilica_of_Our_Lady_of_the_Pillar] in Zaragoza. You feel like an ant when you're inside it.
EDIT:Just to clarify, I've been to all the sites talked about in the above post, as well as others. I felt it would be a bit too obvious to mention Notre Dame de Paris or Sacre Coeur. though.