1) Is "I don't think there's a god" a cause? No.
Then why argue about it?
Non-sequiter. I can think something's true without it being a cause.
If you told me that 2+2 was 5, I would tell you that it isn't, even thought 2+2=4 (or, indeed, mathematics as a whole) isn't a cause.
2) Is it a principle? No.
Then why argue about it?
Non-sequiter. See above.
3) Is it a system of beliefs? No.
Atheism most certainly is a system of beliefs. A very simple one, but a system nonetheless.
It's not a system. It is one absence of a belief. Even if you claim that atheism is a belief (using the logic that "everything is a belief", it's not a system of beliefs. It's one element. Your morals and general life outlook are not affected by it at all.
4) Does it have to be held with "ardor and faith"? No.
Most people, if they are not holding to Atheism with "ardor and faith" are actually agnostic. There's a lot of confusion in the world today about the difference between Agnostics and Atheists. An atheist believes, with no proof, that there is no possibility of the existence of a deity. An agnostic believes, BECAUSE there is no proof, that it is uncertain if there's a deity or not. There's a huge difference between these two, but a lot of people just don't see it.
Wrong.
An atheist does NOT have to believe that there is no possibility of a god. To be an atheist, you just have to not believe in one.
I don't think a God is impossible. I think it's about as likely as invisible fairies in my garden or the FSM, but not impossible. So I call myself an atheist.
Heck, even strong atheism (belief that there's no god) isn't really a religion.
Actually, to be an Atheist you have to actively disbelieve in a deity. If you simply do not care one way or the other and have no interest in the matter, you would be called Irreligious.
Again from m-w.com
Definition of ATHEISM
1archaic : ungodliness, wickedness
2a : a disbelief in the existence of deity b : the doctrine that there is no deity
Definition of IRRELIGIOUS
1: neglectful of religion : lacking religious emotions, doctrines, or practices <so irreligious that they exploit popular religion for professional purposes — G. B. Shaw>
2: indicating lack of religion