I met a member. And he explained to me that they are pretty much atheists but from the point of the law they enjoy religious protections.
Basically if you call yourself atheist, you won’t get any religious protections, because you don’t have any religion. Everyone knows that FSM is a joke, so that wouldn’t fly either. So they created a religion. They chose Satan, to get expected reaction from devoted Christians, they don’t believe in Satan though (unlike wackos from the Church of Satan). You can read their 7 tenets, which sound quite reasonable: thesatanictemple.com/…/there-are-seven-fundamenta…