Warning: The following text contains spoilers relating to the plot of Guild Wars Nightfall.
The temple of the six gods is a very old temple built to honor the 6 gods before the time of Abaddon's fall. According to Kormir and the Tormented Soul in the Gate of Madness opening cinematic, this Temple once stood on the shores of the Crystal Sea, but the Margonites so loved Abaddon that they defaced the other statues (an action that eventually led to the war between the followers of Abaddon, the Margonites, and the followers of the Five Gods) and when Abaddon fell he pulled it down with him into torment, and it is now filled with his Margonite followers.
In the Gate of Madness mission, players venture into the temple in order to clean the temple of Margonites (and Abaddon's generals Shiro and the Lich Lord) and to seek the five gods' assistance in the fight against Abaddon.