This is true and nothing new. Christianity is waning, more in Europe than in the USA, but waning nonetheless.
The Muslim strategy is sound, worked quite well in Lebanon that was once a Christian nation, but the leftists encouraged the immigration of Muslims. Now Lebanon is a Muslim nation, or well on its way to being so.
IMO Europe has been lost to Sharia Law, i.e, too far down that "slippery slope," but there is hope for America.
Not all of Europe.
I predict that within the next 10 years many of the old Eastern Bloc nations, even Poland, will turn to alliance with Russia for their own protection.