http://en.wikipedia.org/wiki/Laicity

I think that sums it up .

To answer Russ' original question, it's the imigrant that must fit to country he's joining. Though he should of course be able to practice religious, philosophical or ideological activities if any, he must respect his new country's laws and culture.

The biggest "mistake" made by imigrants is to try and keep their child in the home country's culture rather than let them be fully integrated in the new one. It's a bigger sacrifice than it seems and I'm pretty sure there would be less imigration if potential imigrants realised that.

Ponrauil