According to the Sacred Scrolls, the Temple of Five was built to recognize five priests who worshiped "The one whose name cannot be spoken," according to Laura Roslin ("The Eye of Jupiter", "Taking a Break From All Your Worries").
Nothing more is told about these priests, or who they worshipped. The being may be a "fallen" Lord of Kobol.
By coincidence or design, the Cylons have five humanoid models that have been accidentally or intentionally forgotten by the active Humanoid Cylon body.
While the Colonials believe the Temple as an artifact of the Thirteenth Tribe, one Cylon model, Number Three, believes it as a place that would reveal the faces of the final five Cylons, despite the evidence that the Cylons did not exist at the time of the temple's construction, 4,000 years prior to the Fall of the Twelve Colonies.
After Three uses the holographic mechanisms of the Temple to reinforce a vision she had experienced before, the knowledge of one face she recognizes is lost again as the other Cylons are compelled to box every one of her model (Rapture).