Pew Research published a detailed study here outlining the alarming growth of the European Muslim population even with "zero migration" going forward. Combined with the declining numbers of the non-Muslim population, it's not a question of "if" but when "when" Europe becomes Islamic.
These growing numbers of Muslims in Europe, combined with the projected shrinkage of the non-Muslim population, are expected to result in a rising share of Muslims in Europe’s overall population in all scenarios.
.... In Europe overall, even if all Muslim migration into Europe were to immediately and permanently stop – a zero migration scenario – the overall Muslim population of Europe would be expected to rise by 2.5 percentage points, from the current level of 4.9% to 7.4% by 2050. This is because Muslims in Europe are considerably younger and have a higher fertility rate than other Europeans. Without any future migrants, these prevailing demographic trends would lead to projected rises of at least 3 percentage points in the Muslim shares of France, Belgium, Italy and the UK.