Another possibility is that religion plays a function than it does in other developed countries is played by nationalism - namely the totalizing ideology that mobilizes people to rally behing the "flag" i.e. the elites. Populations of Germany, Italy, or Japan are ethnically homogenous - therefore nationalism can be used as an expedient fact to separate "us" from "them." In the US, howver, "US nationality" involves a lot of "undesirbales" -such as blacks, asians, eastern europeans, jews etc. which would have to be be included under the rubric of a "true American" had nationality been used as totalizing ideology. Consequently, religion, and more specifically christian identity which is a unique product of american brand of protestantism and exludes both non-christian religions (esp. jews) as well as roman catholics (hispanics, eastern europeans), not mention secular humanists (mostly while folks that also need to be excluded) plays the trick of defining "true Amerikans".
Since religion is often used as a proxy for certain type of in-group solidarity it is both quite popular and brainless - just as nationalism in other countries.
wojtek