Europeans has forgot that Christianity as their root of their culture?

There were many earlier religions before that could be said to be the root of european culture. Many practises of these earlier beliefs were adopted by the later beliefs.

Christianity itself being an imported religion.

it does make me laugh, hearing Christians complaining about other religions becoming a part of the British culture, when their religion had similar origins here.

much as I'd love for the UK to go back to its Pagan roots, as people travel, they take their culture with them, and over time it becomes a part of the wider, multicultural society. in many ways this is a good thing.

It's true that Christianity is the root of European culture. The philosophical and teological paradigm from Augustine, Thomas Aquinas, etc formed the paradigm of nowadays Europe. But i think people begin to bore with the strict paradigm of Christianity. They're searching for alternatives.
I don't think prohibition of hanging cross on the wall is a big problem. I ever heard too that there's a prohibition of using 'jilbab' in the public school.

Christianity is NOT the root of European Culture. Indeed there isn't a unified European culture, different countries and regions of Europe can vary greatly.

Some European countries are trying to keep religious symbols out of schools because of associated problems.

A lot of Europeans have abandonded Christianity because of the various problems it has caused in the past - both on a wider scale and to individuals.

Pope Benedict is worried that people might abandon the church but any organisation that relies on a tax or tithe is only going to keep rich if it has a lot of members. People who abandon a belief do so because it has failed them in some way or other.