Whether ISIS is islamic would eventually be decided in exactly the same way how every question regarding whether any particular new fangled fraction stands for true orthodoxy is always decided - by seeing whether ISIS kill enough of their religious opponents to become the orthodoxy, or whether ISIS is exterminated to the point where most muslims can freely badmouth them without fear of ill effect.
Just look back on Christian history to see how christian most modern christian would have seemed to most christians of the 4th century.
Just look back on Christian history to see how christian most modern christian would have seemed to most christians of the 4th century.