I quote this from a thread on Facebook debating about the existence of God.

You fool. Oh, and I'm allowed to call you a fool, but you aren't allowed to insult me:
“whosoever shall say, Thou fool, shall be in danger of hell fire."(Matt 5.22)

Although interestingly Jesus wasn't much good at following his own rule:

"Ye fools and blind..."(Matt 23.17)”