The dictionary definition of an atheism is as states; "...atheism is specifically the position that there are no deities. Most inclusively, atheism is the absence of belief that any deities exist. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists."
Contrary to this, theism is; "The belief in the existence of a god or gods, especially belief in one god as creator of the universe, intervening in it and sustaining a personal relation to his creatures."
Correct. Atheists and Theists do fight about religious debates such as Christianity vs Hinduism or Judaism vs Atheism. However, saying this does not mean that they are necessarily the same thing. Yes they both believe in something and yes they both fight about similar things, but that doesn't mean they are the same. An apple is round, a plum is round. An apple is sweet, a plum is sweet. But this does not just prove the fact that an apple and a plum are the same thing, similar to your argument that atheism and theism are the same thing.