Atheists, can you please help me? Can 86% of the world's population be religious? I need help to refute this?

I've just read a claim by a minority religious denomination that 'a recent survey found that 86% of the world's population profess some form of religious affiliation'. No source was quoted. I need access to hard, cold facts to refute this allegation. Otherwise they're going to be spouting this stuff to about 7 million people on Sunday! Help!