Bryan Caplan asks (the rest of the post is interesting on other matters):
Seeing a bunch of French cathedrals makes me even more skeptical of the claim (made by Larry Iannaccone
and others) that people weren’t more religious in earlier centuries. If
people weren’t far more religious in the Middle Ages, why did they pour
such a high fraction of their surplus wealth into century-long
religious architectural projects? You could say "It was primarily
rulers, not donors, who allocated the funds," but that just pushes the
question back a step. Were rulers vastly more religious than the
masses? That’s hard to believe. Were rulers trying to impress the
masses by building churches? Well, why would churches impress the
masses unless they were highly religious?
Religious architecture and art were to medieval feudalism what advertising and commercialism are to modern capitalism:
A rather effective way to build support for the status quo using
aesthetics instead of argument. My claim, in short, is that Notre Dame
played the same role during the Middle Ages that fashion magazines play
today. Notre Dame was not an argument for feudalism, and Elle is not an argument for capitalism. But both are powerful ways to make regular people buy into the system.
I would add that churches were a form of fiscal policy and the associated spending was a way to hand out goodies to political allies. (This is especially important if the finished project takes decades or centuries to materialize.) In a time of political decentralization it wasn’t easy to construct or maintain a long distance road. So you had to put a lot of expense in one easy-to-guard place and in a politically correct way. Churches were the obvious choice. Churches may have been an efficient means to store wealth for other reasons as well. If someone is going to plunder you on the run, they can wreck a church but they can’t dissemble and carry away its value very easily.
Robin Hanson might argue that beautiful churches also signaled the status of the elites who built them.