You can understand why so many people in Europe fear the growth in the Muslim population there. While the vast majority of Muslims are peaceful, within that population are a few murderous fanatics who reference Islam as an excuse to engage in virtually any kind of barbarity. (See ISIS for what they are capable of.)
And even in the wider Muslim population there tends to be considerably less tolerance than among native Europeans of other groups' and individuals' views and a disinclination to integrate with the wider, democratic and tolerant society that the West is so proud of. Of course, the West's record of tolerance is erratic, and most literate people know the totalitarian viciousness that it has been capable of. Still, it has generally been the freest and most humane part of of the world for the past few decades, while Islam's golden age of tolerance was a lot longer ago.
The trouble is that so many Muslims move to the West (Western and Central Europe, the U.S., Canada, Australia and New Zealand) only for economic reasons and/or to escape the brutalities and corruption of the Mideast, South Asia and North Africa, where Islam is dominant. Too few move to the West primarily to enjoy its respect for tolerance and human rights, which too many Muslims don't seem to understand at all.
Islam is a far more encompassing religion than Christianity has been for a long time. It's been quite a while since the term ''Christendom'' was used widely. Among other things, traditional Islam sees the state and religion as the same thing. So there's a totalitarian potential in parts of Islam that threatens Western society. Europeans know that, and that's a major reason so many oppose further immigration from Muslim nations.
-- Robert Whitcomb