As for Shia and Sunni coming together, it's possible though a bit unlikely... They've not been on good terms for quite some time, even Catholicism, Orthodoxy and Protestantism failed to come back together after hostilities died down.
Even if they have the Sunni/Shia division, the Muslims will unite when they face a common enemy. Have you ever heard of any Sunni Shia clashes in countries where Muslims are minority? These clashes only happen in nations where they are more than 90% of the total population. The same goes with Catholicism/Orthodoxy/Protestantism.
Well maybe they might not split along sectarian lines but they still do clash sometimes, like Turks and Kurds in Germany. Many of the fighting inside refugee camps are also based on ethnicity. Those that are already in Western countries might see the natives as a common enemy but the gov'ts of the countries themselves would have a harder time coordinating. They are more likely to use demographics than band together and try to take over a non-Muslim country.
Sigh, the Europeans are gonna lose this. Pretty much all of them have a dwindling population. It seems only Spain is receiving non-Muslim immigrants, mostly from Mexico. Maybe they should start taking in Christians from former colonies?
I'm also not seeing Christians band together. Save for the Catholics, the other sects are rather split up and without hierarchy. And it's not like the Pope is gonna call for a Crusade anytime soon.