Over the years I have come to laugh at the assertion, but it is nonetheless disgusting. Imagine, Saint Peter's Church is not Christian?!My favorites are the people who claim Thomas Jefferson was a Christian (despite his disbelief in Christ's divinity) but would deny Roman Catholics the same status. Of course, if Jefferson and Franklin were not "Christians", then the entire "Christian Nation" premise falls faceward.
In the US at least, Jews and Catholics do not seem inclined to prosletyze, let alone legislate their creed (its a lot easier to buy pork in a Jewish neighborhood or a burger on a Friday during Lent in a Catholic one that alcohol in certain Baptist areas). I'd consider that a sign of personal religious security. :)
-Eric