Here's a fictional scenario that highlights the folly and danger of faith:
A deeply religious man starts hearing a voice in his head. The voice tells him that it is God. "God" tells him that it would make a great nation out of him if he obeys his commands.
The man tells others about it, but they don't believe that it's really God talking to him. He doesn't seem crazy though, everything else about him is normal except for him hearing the voice. He develops strong faith that it really is God that is talking to him.
One day he has a terrible seizure followed by a coma that lasted a couple days. Doctors scan his brain and discover a tumor in his brain. Because of the location of the tumor, doctors know that it is the cause of the auditory hallucinations the man has been having.
They inform the man but he refuses to accept it. He has faith in his "God". By faith, he perceives that the doctors are misled by Satan. He perceives that they were sent by Satan in a bid to destroy his faith. It was a demon that entered into him and gave him the seizure. The whole incident was all an elaborate machination of Satan to get the man to renounce God! This he perceives clearly, by faith.
Later, "God" speaks to the man and asks him to sacrifice his three year old son to him. His wife and son come to the hospital to visit him shortly after this and when his son hugs him, the man, by faith, grabs his son by the head with both hands and snaps his neck.