Is Satanism a religion? Answer, no. Religion is a group-orientated outlook with a fixed set of beliefs, rituals and hierarchy, where individuality is sacrificed in favor of a collective worship. Satanism focuses on individuality, so it might be a philosophy for those that view Satan as a symbol, or a spiritual outlook for those that view Satan as an entity.
Satanism is a Left Hand Path (LHP) outlook where the focus is upon the individual, this is in contrast to Right Hand Path outlooks such as Christianity which focus on the group at the expense of individuality. The Church of Satan is also an example of a Right Hand Path outlook, even if it claims to associate with the LHP and rejects religion, this is because it is a group-orientated organization with the same fixed baggage as a religion such as its dogma, rituals and hierarchy.