To me, the continued spread of the Muslim religion is concerning. I'm not religious, but I see the principles of Islam to be extremely regressive, IE women's rights/gay rights/etc.
Christianity is diminishing. "It found that the percentage of Americans who described themselves as Christian had dropped from 78.4% to 70.6% between 2007 and 2014."
http://www.bbc.com/news/world-33256561
Meanwhile, Islam is spreading.
http://www.mirror.co.uk/news/world-news/islam-fastest-growing-religion-overtake-6986333
So, in some respects, the fear of Islam is justified.