A frog lives in a well. It's been there its whole life and knows nothing of the world outside. One day, a turtle falls into the well and tells the first about the Blargleglarb ocean. The turtle tells the frog that the Blargleglarb ocean is much bigger than the well, that it shimmers in colors of grue, bleen, and violellow, and that great Flignab beasts swim in it. The frog has no way of comprehending the size of the Blargleglarb, or what grue, bleen, and violellow look like, or what a Flignab is, because they are too far beyond his experience.
But he believes it all, because the turtle tells him that C'thulhu, the only true authority, said it was so. And after all, the frog can't remember every second of his life, so that proves it.