I seem to have ticked off a lot of people with my last post, so let me clarify a couple things.
I believe in religious inertia. This is not a scientific theory, just one I've cobbled together based on my observations. There are people who defy the laws of religious inertia; I am simply trying to explain the behavior of the majority of people.
Ned's Laws of Religious Inertia
1. Most people do not change religions during their lifetime.
2. Most people are not interested in changing their religion.
3. It often takes a shock to the system or a fundamental change (e.g. marriage to a spouse of a different religion) to break someone out of their religious orbit.
4. Strong social forces like families and friends work to keep people in their existing religious orbits.
I don't think these laws are very controversial. Obviously there are lots of exceptions, but I think that this is generally how the world works.