Islam will never take over Europe. First will come civil war and then ethnic and religious cleansing.
Islamists could never hope to conquer Europe, the geo-politics are against them:
1) Even if Muslims became a majority in Europe, it is doubtful they could legally overturn 'bills of rights', constitutions, or violate European Union membership requirements.
2) Russia has territorial ambitions in the Baltics, and there are large Russian populations across Europe. Russia wouldn't stay out of a major conflict in Europe, any more than NATO states.
3) Any conflict with Islamists would not be evenly matched, if the conflict between Serbia and Bosnia, Kosovo,etc is any judge.
4) Much of Muslim population is from Turkey, refugees fleeing Islamists and war, or from well educated Middle Eastern families.
5) Recruitment rates are low for committing terrorism or engaging in an insurgency internally in Europe. Though European countries should keep an eye out for returning Islamists, and either refuse them entry or imprison them if they return.