Was it a small group of humanoids that came from another star system composed of 12 colonies after being pursued by a society of artificial lifeforms that was bent on their extermination?
Her destiny ( teased throughout the series ) was to sacrifice herself to lead humanity to salvation ( earth ). It's similar to Christ in a way - self-sacrifice and the second coming to lead the faithful to the kingdom of god.
I think from day one of her existence, she was a kind of power on the same level as Baltar and Six's angelic alter egos. Watching it the first time when it originally aired, I had thought she was one of the Five (even though part of me thought that was too obvious, and a cop out)... and then the fifth member was revealed and it wasn't her.
Clearly the writers had given her a role beyond merely human or Cylon, something part of the grand scheme of God that Messenger/Head Baltar and Six were part of.