My first GnK game, I got a shiny new prophet, and sent him over to Ghandi to start converting the heathens to my new religion. Ghandi, however, didn't like me stealing away his hindus to christianity. He never forgave me that for the rest of the game, which went over 1100 turns. Moral of the story- economic and military insults may have a limited shelf-life, but mess with somebody's religion, and they NEVER forget.
Memo to Ghandi: If you hadn't been such a turd to me for the next 800 turns, I wouldn't have been forced to raze two-thirds of your cities and pillage all of their improvements too. Just sayin.