To the original post, no there is no such thing as an atheist. We are given two choices. God or Satan, there is no in between, you are for Him or against Him.
This is a false dichotomy. Let's look at some other possibilities:
1. You can be for God and against Satan.
2. You can be for God and not believe in Satan.
3. You can be against God and for Satan.
4. You can be against God and not believe in Satan.
5. You can be against God and Satan.
6. You can be somewhat for God but not support him entirely while believing in Satan.
7. Same as 6, but also not believe in Satan
8. You can not believe in God or Satan.
To be an atheist as it is defined is to serve Satan.
I'm sorry, but that's NOT the definition of atheism.
An atheist is a person who doesn't believe in any higher entities. If you believe in God, you're a theist, NOT an atheist. If you worship Satan, you're NOT an atheist - you're a theist.
I care too much to lie. To be a Buddhist is to serve Satan. To be a an Islamist is to serve Satan. But, even some so called Christians serve Satan and don't even realize it.
Even if this is true, and anything but worshiping God in a very particular way is to serve Satan (including lacking belief in God), this doesn't change the fact that atheism has nothing to do with whether or not a person serves Satan. It defines whether a person believes in a higher entity or not.
The same is for atheist, they think they are serving themselves, but spiritually and in reality they are serving their father Satan.
What do you mean by "father"? Atheists don't believe in Satan, therefore Satan isn't their "father" or their "master". To suggest he's either one would also suggest atheists believe in Satan - which is literally impossible because such a belief, by definition, would make them theists. You can argue that atheists serve Satan without realizing it, without believing in him, but be careful not to turn around and act as if atheists believe in Satan and serve him.
Lets keep our definitions as clear and concise as possible. If being an atheist leads to serving Satan unintentionally, keep in mind that serving Satan is still not a part of the definition of atheism. To be an atheist, you also need to be alive, but atheism isn't "A lack of belief in god and living".