The One Whose Name Cannot Be Spoken
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
The Sacred Scrolls notes that the Temple of Five was dedicated to the five priests who worshiped "the one whose name cannot be spoken" (The Eye of Jupiter).
Nothing else is known of this being.
However, the belief by the Number Threes that the Temple (constructed by the humans of the Thirteenth Tribe over 4,000 years prior) would reveal the identities of the final five missing humanoid Cylons, as well as the temple's connection between Kara Thrace and the Eye of Jupiter revealed by the Temple's sun strongly suggests that this being was related to the Lord of Kobol, or the Cylon God.