…a “high” migration scenario projects [a] record flow of refugees into Europe between 2014 and 2016. (…) In this scenario, Muslims could make up 14% of Europe’s population by 2050 – nearly triple the current share, but still considerably smaller than the populations of both Christians and people with no religion in Europe.
(…)
The refugee flows of the last few years, (…) are extremely high compared with the historical average in recent decades, and already have begun to decline as the European Union and many of its member states have made policy changes aimed at limiting refugee flow.
Predicting future migration levels is impossible. (…) if regular migration continues at recent levels, and some asylum seekers also continue to arrive and receive refugee status – but not as many as during the historically exceptional surge of refugees from 2014 to 2016 – then the share of Muslims in Europe’s population as of 2050 would be expected to be somewhere between 11.2% and 14%.

While Europe’s Muslim population is expected to grow in all three scenarios (…) Europe’s non-Muslims, on the other hand, are projected to decline in total.

Between mid-2010 and mid-2016, the number of Muslims in Europe grew considerably through natural increase alone:

Three scenarios illustrate what the Muslim population could look like in Europe in 2050 (Methodology). Résumé en langue française ici, texte intégral en anglais là.