As per Wikipedia:
Satanism is a group of ideological and philosophical beliefs based on Satan. Contemporary religious practice of Satanism began with the founding of the atheistic Church of Satan by Anton LaVey in the United States in 1966, although a few historical precedents exist.
Atheism, in the broadest sense, is an absence of belief in the existence of deities
The mere fact na may mga Traits ni Satan na gusto nilang i-achieve, that would only suggest that they acknowledge the existence of Satan and is following his steps. Jan palang po eh tanggal na sila sa qualification to be called an Atheist.
Simple lang. If you are in a religious group and practice what they preach you are for sure NOT an Atheist.
Pero di po natin iniinvalidate ung mga taong patago na Atheist. Yung, they are in a religious group but does not really believe in its teachings and does not acknowledge their "God" pero di malabas kasi religious and family.