I feel that Satanism can be considered a religion, but that it doesn't have to be. That clearly, there are different sects with some treating Satanism as more of a philosophy without believe of supernatural beings or gods… So I'd yes, but also no. If that makes sense.
There are those, no matter how crazy they may seem from a religious or secular view, who honestly believe in and worship Satan as an actual being… while others, see him as more of a philosophical concept embracing intellectual thought and freedoms, or other such things.