Atheism: The denial that there is a god.
This Philosophical Dictionary is atheistic, which is to say that its writer does not believe in any god, and indeed never did. The reasons for this belief are given in the lemmas on Religion, Faith and God, and the links there provided to related items.
The reasons for not being agnostic are given in Agnosticism, but may here be summarized by noting they are quite similar to the reasons for not being agnostic about dragons, elfs, witches and mermaids.
One popular fallacy about atheism is that the believers in some religious faith tend to insist, and perhaps believe, that atheism is "as much" a faith as their own religious faith. This confuses, willfully or not, a belief in a presence and a belief in an absence, and insists that the belief in an absence is a belief in a presence, perhaps in the presence of an absence. This is just a stupid confusion if it is not dishonest: If there is no food in the larder, it does not help to insist that then there must be something to eat, viz. nothing.
Also, since one can prove empirically that (Ex)Gx (there is an x which is G) simply by showing a particular a to be G (Ga), even though this may be difficult, whereas one only can prove empirically that ~(Ex)Gx (nothing is G) by showing that (x)~Gx (every thing is not G), it is up to those who claim existence of a G, to prove that is so, or at least cite evidence why - they believe - that this is so.
And this holds for gods, geraniums and griffons alike: Until Ga, there is the presumption that ~(Ex)Gx, or else one begs the premiss (Ex)Gx i.e. assumes what one should prove or at least support by evidence.