As I understand it, Satanism would have to be a sect of anti-christianism and thus a theology. Am I wrong? Please explain.
Depends on what sort of Satanism you're talking about. The most active varieties are almost parody religions that don't actually believe in a personal devil. Atheists have been known to join up just because believers claims atheistm = satanism so, you know, it's that counter-factual, I'll-show-you sort of thing going on.
If I recall correctly I'm talking about LaVey Satanism and The Satanic Temple which I think is an offshoot of that, historically. Anton Lavey was a provocateur and the religion has that sort of ethos.
My favorite thing about them is that they are very effective in church / state separation issues. Seems like every time some fundamentalist wants a statue to the Ten Commandments on public property like the local county courthouse, there's the Satanic Temple applying for a Baphomet statue to go right along side it. The only way, legally, they can prevent that from happening is to prohibit all groups (the general legal principle is either all beliefs are equally represented in the public square, or are equally excluded). Another example is so-called "release-time classes" that some evangelical groups have, where middle school or high school students are taught Bible lessons so long as it's off-premises and non-compulsory. So along comes the local Satanists with a request to have a Satan Club along the same lines. And the release-time class goes away.
Brilliant and effective strategy. They're not my cup of tea but I gotta like them just the same.
Not all sects are the same!
[thesatanictemple.com]
You must believe in God.