Rodorsceaft: Pillar of Heaven
Any person with ranks in Knowledge (arcana), Knowledge (religion) or Knowledge (nature) who learns of a particular ritual may attempt it; having a caster level is not always necessary. Rituals are long, tend to be quite complicated, and require focus and dedication and usually hard-to-find ingredients that are used up in the ritual.
Every ritual has a minimum Knowledge skill required to undertake it. This abstracts all the “small knowledge” that goes into making a ritual, things the designers wouldn’t necessarily have written down: the way to arrange things on an altar, the right clothes to wear, the mindset, etc.