The One Whose Name Cannot Be Spoken
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The Temple of Five, also called the Temple of Hopes by Ellen Tigh, was originally believed to be built by the Thirteenth Tribe. When the Thirteenth Tribe is revealed to be a Cylon race (TRS: "Sometimes a Great Notion"), the nature of this being becomes more apparent.
The Messenger Gaius Baltar, roaming the streets of New York City 150,000 years after the fall of the Twelve Colonies, chastizes the Messenger Number Six for using "God" as the name of this being, indicating that it did not like the use of that name (TRS: "Daybreak, Part II"). Therefore, "God" may be the Name that Cannot Be Spoken, but it is not entirely certain.