I would be worried about any rapidly growing religion that opposes some of the core principles of my society. Currently Islam fits this description in Sweden. Publish stuff they consider against Islam and you will be a marked man. The Jews are being driven out of Malm?. Many Imams here advise Muslims not to go to the police with what they consider religions matters (but things that are clearly violations of Swedish law). Now, every religion has it's crazies, but when the Muslim community refuses to speak out against them it is easy to get the impression that they are silently agreeing. I wouldn't say that Islam is a big problem today in Sweden, but unless we somehow manage to enforce our laws it could definitely become one in the future. I should also mention that the riots of the past week in Stockholm probably doesn't have any connection to Islam.