the west is not defined as adjunct to or derived from christianity, but rather arose in a dialectic against it. contemporary western culture is the literal ideological opponent to historical christianity, in every conceivable way.
western culture embraces science, rationality, discourse and empiricism. these are the values that make us what we are; they define us, they explain us. christianity is the (foreign) system we had to overcome and dismantle in order to get to this point of defining ourselves in opposition to it.
a collapse into a dark age would be a return to the christian tyranny that we escaped, not a retreat from a christian identity that we've already discarded.
stated simply, muslims don't scare me because they're going to overthrow the christian order; muslims scare me because they're pretty much exactly the same thing as christians, and threaten to bring the christian order that we eradicated back, again.
and, what i'm really afraid of - what i'm really opposed to - is the return of christianity. muslims are just a newer, scarier type of christian.
a pile of shit by any other name will smell just as bad.