Your magic is tied to belief, and your belief is tied to a code of behavior. You must follow your ethos, or your powers fail. It does not matter if you follow a god or a philosophy; what matters is that your ethos and tenets are central to all that you do.
This is the basis of all divine power; if your god actually grants you these magic, or if your faith is just a psychological crutch you use to balance your powers is an open issue. What matters is that your powers start to fail if you break the tenets of your dogma.
Most dogmas have a built-in antithesis; there are opposing gods, devils, the dark side and other negative, opposed forces in most mythologies. If you fall from the path of righteousness, you can usually pick up and continue your studies under a new master, but this will mandate a drastic change in role and personality, and may turn you into an NPC.