I received the quest in which I am charged with taking my religion's holy city from the civ with which I was at war.

It said I would fail the quest if peace was declared, if another civ captured the city, or if I changed state religion. Neither of these things happened. I captured the holy city, completing the quest perfectly. Yet, on the next turn, I was informed that the quest had failed.

A bug, if I ever saw one.

It looks like it's fixed in the upcoming patch.

See here for more info:

http://forums.civfanatics.com/showthread.php?t=243264

I'm afraid I can't see it.

I'm afraid I can't see it.
Its a long list :)
Fixed Crusade event

Its a long list :)

Ah. I only did a search for "quest" :)