Doug makes the excellent point that nationalism can be wrong if it makes the nation an idol. This is a failing all conservatives are vulnerable to (Postmillenial or Dispensational).
The most interesting point:
"The churches were full before 1914, half-empty after 1919, and three-quarters empty after 1945" (quoting Peter's book)Now, there are many effects underlying this.
Doug and Peter associate it with the idolization of the nation (confusing God and nation). That defects in the worship of the nation were applied to people's notion of God, causing them to turn against God.
There are two other factors:
- Modernism - Modernism began much earlier (Oden says 1789), but it really started to peak in the late 1800's. Prior to the World Wars, the American Civil War (1861-65) could be dismissed as an outlier (even a necessary good, eradicating slavery). However, the World Wars put an end to any thought that mankind could do good on its own (or at least, gave it a sound drubbing).
- Postmillennialism - It is well said that the World Wars killed postmillennialism (PM). PM preceded Modernism, and ideologically had little in common.
Modernism is pretty soundly defeated (having given way to Post-Modernism, which is also making its way into many churches). PM is said to be making a comeback, although I have seen little of it.