The Hierophant represents spiritual beliefs, institutions, and traditional wisdom. So, naturally, Celestia made sense for this one because she's ruled Equestria for centuries. She *is* the institution.

The Hierophant's triple cloak and three rings of light represent the conscious, sub-conscious and super-conscious mind. The gesture she makes with her wing is meant to represent the connection of the heavenly and the mundane, the sky and earth. The two ponies before her are there to take in her ancient wisdom, mysteries that only she can unlock, as is represented by the keys she wears around her neck.