The Ancient One is the Sorcerer Supreme, and Kaecilius rejects the idea that she’s the only one who can be that powerful.
Kaecilius isn’t exactly buying what the Ancient One is selling.
Kaecilius is already a master of the mystic arts before the film begins, but he knows that there’s more to learn.
When asked by Marvel.com why his character in the supernatural blockbuster rejects the teachings of the Ancient One, Mikkelsen’s replies that he didn’t reject the teachings; he simply wants more of them.