The Celestial Law is what the Valor Santori and all of it's protectors uphold. Therefore breaking it in their presence can result in them taking hostile action on behalf of the Celestials.
Innocent life is to be preserved at all costs.
Threats from outside the Mortal Realm hold priority in all duties.
The Celestials must guide and advise the Mortal Realm, but not shape it's destiny.
Magic is as sacred as life, the knowledge surrounding it must be gathered and protected.
Darkness must not be feared, but kept in the right hands.
Time and Fate must not be tampered with by anyone but the Guardians of Time.