According to everything I've read, the Cathedral of St. John the Divine is the largest cathedral in the world. Located at the 112th street and Amsterdam Ave in NYC, the Cathedral is still far from finished. On the inside the cathedral recently hosted an exhibit on police violence and brutality that was truly stunning. In the past, Buckminster Fuller thought of building a bucky-ball around the center of the cathedral. Also, the author of the beginners guide to constructing the universe has done stonework there.
When I was a freshman (or first-year) at Columbia University my room in John Jay Hall overlooked Amsterdam Avenue and the cathedral. A beautiful sight to wake up to regardless of one's religion or lack thereof.