According to Pew:
While Muslims are still a relatively small share of Europe’s population (roughly 5%), they are set to continue rising as a percentage of Europe’s population, according to a new Pew Research Center analysis of demographic data. This is true even if immigration stops entirely in the coming decades, which is a highly unlikely scenario. And if migration to the continent continues at medium or high levels, the share of Europe’s population that is Muslim could more than double between now and 2050.
Also, please see Pew’s 5 facts about the Muslim population in Europe.