It is forbidden in Islam to declare people non-Muslim unless he...

It is forbidden in Islam to declare people non-Muslim unless he (or she) openly declares disbelief

I can't help noticing this declaration means all protestation - often by other Muslims - that ISIS aren't true Muslims must therefore be invalid. ISIS have never declared themselves to be non-Muslims therefore they must still be true Muslims. Therefore their actions must be those of a true Muslim.

QED.