There are many people who describe themselves as Atheist. Then there is the definitions they are labeled with, which often do not match. So, here is what I consider the accurate definitions of various theistic types.
Theist: Believes that there is some sort of Divinity.
Atheist: Has no religious belief system.
Anti-theist: Believes that there absolutely no sort of divinity at all
I am not an anti-theist. There is no definitive way to prove that there is no sort of divinity at all in all of existence. Lack of proof is not proof of lack. This applies to both sides of the coin. There is no proof that there is a god, there is no proof that there is NOT a god.
Now, I will say that I do not believe that there is a god that literally matches any current religious document. The old pantheon religions tend to speak of deities that live specific places and have specific forms, etc, and the worlds described there-in do not exist.
But this can be said of any religion based off the Torah / Old Testament as well. The sky is not a Vault above the flat plane of the earth, there are no Waters Above, and we've established solid evidence that humans evolved from other creatures and that the universe is a lot more than 6000 years old. So basically, there is no God who did the things specified through out much of the bible, because the events there in have been disproven.
Scientology fails this test too. In addition to the lack of evidence of any of this alien activity they claim exists, specific devices they claim prove their claims, are bogus (as per this YouTube clip on the E-meter).
So there is my specific disbelief in religions as so far claimed & described by man. But, if you ask this question, things change: "In or beyond this universe, does there exist an entity (or multiple entities) so far beyond our comprehension, understanding, and power that it could only be considered God or A God?"
I have to say, in all fairness and because I have a scientific frame of mind, to say "I don't know." Mind you, I don't consider it probable, but it is not impossible.
The who believe ardently that there is no such thing as god, that it truly is impossible for such a being to exist, are Anti-Theist. And while their view points may be inspired by the findings of science, in the strictest sense it is not scientific, as they believe in something which is not provable.
But one should also consider the arguments represented Here and Here, which make good points about how one who makes a claim about the positive existence of something needing to be the one who presents Positive proof.