And no, Martha, it's not Osama:
"What does Jesus say is the most dangerous thing in the world?
First, Jesus tells us not to fear physical death. “And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell.” [Matthew 10:28] Physical death is not the most dangerous thing. There is something worse: judgment and condemnation. This is to be feared.
This is why Jesus teaches us the most dangerous thing in all the world is false teaching. That's right: false doctrine. I'm sure that if I did a survey of a thousand people, asking them the question: What's the most dangerous thing in the world? that the answer 'false doctrine' would never show up; that is, unless we ask Jesus.
Just about every time Jesus (and in fact the entire New Testament) uses the word 'Beware' He is using it to warn against false teaching and false doctrine."
You can read the entire piece here.