In the UK there is 'Practicing and Practising'. This clarifies the differences more. I always remember this a 'Practice with a C for Clinic, as in Doctor's'.
I think you are getting confused between 'To do or perform (something) repeatedly in order to acquire or polish a skill' and 'To carry out in action'.
There is no such thing as either a 'practicing' or 'practising' Satanist.
"...I Myself am Heaven and Hell"