The Five Priests and the Final Five
On the Algae Planet, we encounter the Temple of Five which we are told was "built for the five priests who worshiped a god whose name must not be spoken." We're not told why his name must not be spoken, but a deleted scene described the story of a lord of Kobol who was known as the "jealous god" who wanted to be ahead of all the other gods. Both of these attributes, of course, seem patterned after Yaweh from the Torah, and many think both quotes refer to the same lord, who may also be the Cylon's god.
Tyrol, secretly a member of the Final Five Cylons, was taught about this temple as a child, his parents being a priest and an oracle. And when he came to the planet, a secret compulsion to find the temple was triggered, in addition to a compulsion to protect it -- he disobeyed direct orders to destroy it. The temple contained the same design implanted into Starbuck's brain as a child, part of the destiny that Leoben told her had "already been written." That design matched both the nova of the Algae Planet's sun, and the Ionian nebula which, it turns out, is the trigger location for the 4 sleeper members of the Final Five on board Galactica. At the same time the Cylons are also drawn to this temple by Baltar's interpretations of the "ravings" of the Hybrid, whom some of the Cylons believe communicates directly with their god. (She also refers to the "five" lights of the apocalypse in her ravings.) They also believe the temple was built by the "13th Tribe" (ie. Earthlings.) As both Baltar and Gaeta point out, it's way too bizarre to be a coincidence, all this was planned, and as far as can be told, planned thousands of years ago.
Now my own dramatic intuitions immediately saw a connection between the Five Priests and the Five Cylons. Quite simply, there are almost no coincidences in fiction, and the identical number seems to be telling us something. That was just an intuition, but of course when the temple is activated by a process planned thousands of years ago, D'Anna receives the vision, and it's the final five Cylons. (Cavil seems to know this is what she will see and is ready to shoot her to stop it.) Interestingly, the five glowing figures stand on 5 out of 6 drapes, with one empty drape. As yet there is no clue who the 6th drape is for. It might be their god, or the chosen one or something else.
These 5 glowing figures appear in this vision, and in later visions shown in the Crossroads episode within the intact Kobol Opera House. We don't know the date, but presumably this opera house was intact only in the period from 2,000 to 4,000 years prior to events in the show.
Clue after clue tells us the Final Five are old. Very, very old. While one can imagine that they simply repurposed the 4,000 year old Temple of Five, their fondness for the ancient Opera House confirms their age -- and thus makes it highly unlikely they are not also strongly connected to the original source of the temple.
Yet some ask, how can the characters of Tigh, Tyrol, Foster and Anders be 4,000 or more years old? Well, those bodies aren't, but they are full Cylons, producer Ron Moore tells us. And Cylons, we know, can create copies of themselves which believe they are human. The only other copies we have seen (the 5 glowing figures) so far appear only in visions, but these shared visions are coming from somewhere, so we have to presume that fully aware copies of the Final Five are around and following their own agenda. Indeed, there have to have been aware copies to make the sleeper copies that have been with the fleet. The "Significant Seven" certainly don't seem to have been the ones to make those copies -- they are superior biotechnology, much more closely human and even able to age and easily breed. The only other being we've seen who might have made them would be the Cylon god itself.
So right now I rate it as highly likely that the Final Five are the Five Priests, the jealous god who must not be named is the Cylon god, and they have been manipulating events, including inserting sleeper copies of themselves into human society since long before the creation of the robotic Cylons. A less likely, but possible scenario involves the Cylon god doing all this work, and inserting these five at the appropriate time. But in choosing between these scenarios, it the coincidence of "five" seems to point strongly towards the former.