I am an Atheist and see very little difference between Atheism and Satanism.
This is like saying "I see little difference between a flower and a tulip". The difference is very simple. There's no need to over-complicate it.The word atheism means not believing in a deity (a + theos + ism = "without" "deity" "belief"). That's ALL the term means.
Period. If you don't believe in an deities, then you are an atheist. Satanism is a non-theistic religion, and one that explicitly asserts that deities are the invention of the human imagination. Therefore, Satanists are atheists too.
Perhaps you've been duped into letting Christianity define "religion" for you, erroneously thinking that "atheism" means "no religion" (it doesn't), or believing that theism is a necessary component for religion (it's not). There are already plenty of atheists who practice some sort non-theistic religion: Theravada Buddhism, Taoism, Raelianism, Church of Scientology, Unitarian churches who include atheists, and so on.
"Atheism" is not a religion for the same reason that "theism", "monotheism", "polytheism", and "deism" aren't: these words only describe ones stance regarding the existence of deities. Merely saying "I am an atheist" doesn't tell you anything about that person's philosophical views or convictions. Needless to say, you can be an atheist and not a Satanist, just as you can be an atheist and none of the non-theistic religions I listed.
If my understanding of your essay is correct, the belief of an all powerful person would indeed make Satanism a religion and not a form of Atheism.
No, what makes Satanism a religion is the fact that it includes the main components of a religion: philosophy, ethics code, dogma, ceremony, symbolism, and so on. Once again, theism is not a necessary component. The fact that we take a symbolic view in being "our own gods" (read: still not theism) is beside the point.