The history of Westeros is written, in-universe, by the maesters of the Citadel. Like all historians, they’re not infallible. In particular, the maesters are focused on rational thought, evidence and reason, even in the face of evidence of magic in the world (such as dragons.)
Some of Westeros’ historians though are more open to the possibility of the arcane and inexplicable. In particular, the works of Septon Barth are referenced by others, especially in TWOIAF, usually so that the ‘author’ Maester Yandal can then scorn the theory. Which has led to the popular idiom amongst fans: “Septon Barth was always right.”
It’s GRRM’s way of telling us the truth, while showing us that the people of Westeros don’t believe it.
So what has Septon Barth actually said?