Too bad the priests and noblemen weren't in on the joke.
The Third Crusade.
The letter's message that God would finally step in to take non-Christians to task resonated heavily throughout Europe, which suddenly remembered it still had an ax to grind with the Muslims after the unmitigated disaster that had been Crusade 2: The Crusadening. As such, many notables clung to the letter's messages, and it ended up being one of the deciding factors that whipped up European nobility enough to have another crack at reclaiming Jerusalem. What could go wrong?
So the Third Crusade kicked off -- you know, the one where Orlando Bloom fights Saladin -- and proved successful enough for Europe to launch another six of them over the next century, resulting in immeasurable suffering and death.
The original example of a poorly thought-out trilogy leading to a bunch of shitty spinoffs and sequels.
Thanks, random dickhead in Toledo!