Think about this nightmare scenario.
Millions of Muslims, mainly young men of military age, all angry and aggressive and demanding have already been allowed in the most ORDERED fashion to arrive on the Greek Islands, MOST without passports or ANY type of documentation, then in the most CHAOTIC fashion have charged across our ENTIRE Continent demanding to be allowed to stay or else.
If only 0.05% of them are ISIL/ISIS/Daesh, Al-Nusra or Al-Qaeda, that's a lot of terrorists.
Now say if 100 of them are told to spread into say 6 European nations, because of the stupid Schengen Open Borders policy, and then on ONE day ALL at the same time they blow themselves up in suicide attacks....that's 100 suicide bombers on ONE day across 6 nations.
That would create a nightmare situation of total chaos and mayhem and carnage, resulting in 6 European nations and probably the whole Continent being put in total lockdown with a Continent-Wide State of Emergency and Martial Law....the ramifications of this are very frightening, because history shows us that once Martial Law has been declared it doesn't tend to get undeclared, so you're then into a full-on 1984 Orwellian situation.