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 (TRS: "The Eye of Jupiter", "Taking a Break From All Your Worries").
Nothing more is told about these priests, or whom they worshiped. Given that the passage was found in the Scrolls, the being may be the one god worshipped by monotheists in the Colonies and the Cylons or some other historic or spiritual figure.
While the Colonials believe the Temple is an artifact of the Thirteenth Tribe, one Cylon model, Number Three, believes it is a place that would reveal the faces of the Final Five Cylons, despite the Cylons not being created until 4,000 years after the temple's construction. The later revelation that the Thirteenth Tribe were Cylons predating those of the Twelve Colonies strengthens the connection.
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 (TRS: "Rapture").