Yoshie, that's probably true, but excluding people from in-group based on their ethnicity alone would sound to obviously racist - and good christians want to avoid bad publicity. Do not also forget that christianity can be a vehicle for some 'foreign elements' - such as the moonies - to be included in the in-group of 'true americans' regardless of their immigrant-asian status.
Do not also forget the instituional aspect of american religion - which tends to be very parochial and local - in contrast to the universalistic ambitions of the roman catholic church. that combination of institutional localism and absence of overtly ethnic identity makes american religion an extremely useful tool for de facto fascist movements and groups without making them appear blatantly fascist and racist.
Again, it was easy for the germans to built a fascist movement on national identity alone because that automatically exluded all undesirables: "bolshevists" (russian nationality), jews, gypsies and later virtually every other european nationality they conquered (unless one opted to be "included as a volksdeutch - ie. demonstrated his "ethnic connection" to the german nation). You certainly agree that such an approach would be very difficult to implement in this country - because of its multinational character. religion does a much better job becaus it exludes many native-born american who are 'undesirable' and at the same time makes it easier to be a 'volksamerikanische' all that is needed is a 'spiritual conversion' to the right brand of the local religious cult (e.g. moonies).
wojtek