- Subjectivity of evidence- The only evidence that is offered for the existence of a deity is that of either seeing design in the universe (necessitating, for some reason, their particular flavor of creator god) or of having "experienced" God. The fact that people of all religions claim to experience their particular deities, or have visions consistent with their religion's cosmology, is not a very good indication of truth. Especially considering the rarity of such experiences, and the fact that they could be more reasonably explained as credulous hallucinations or even willful deceit. Without hard, verifiable, objective evidence consistent with a particular view of the divine, I am left unconvinced, but not immovable.
- Inconsistency with reality- There are quite a few religions that offer a perspective on existence that simply contradicts known facts, and the rules of logic. Specifically, the Christian God is internally inconsistent if you accept that he allows free will, is omniscient, and omnipotent. And the religion is externally inconsistent if you accept that he is omnipotent, omnibenevolent, and acknowledge that there is injustice and suffering in the world which is NOT necessarily alieviated in their view of the afterlife. In addition, the Bible is also externally inconsistent with reality, in that God does not interact with people in the explicity manner portrayed in it. These contradictions, both internal and external, are actually a very minor consideration, but one worth making.
- The unknowable nature of the supernatural- We are natural beings, in a natural world, confined to natural senses and only able to observe natural phenomenon. Supernatural beings and phenomenon are simply beyond the scope of our observations and are unknowable (being, by definition, beyond the natural). As such, we can never have objective evidence of a truly supernatural entity and, in our experience (reflected in point number 2) we have no reason to believe that such an entity has any influence on our existence that is distinguishable from regularly occuring natural phenomenon. As such, any claims pertaining to specifics of the supernatural are impossible to verify.
- Speculative traits and vague principles- This is not a reason for disbelieving in a god, in of itself. It is a reason to cast doubt on any of the imagined forms they are claimed to take. When people attempt to prove that a god must exist, they tend to make arguments regarding first causes, moral law-givers, etc. The problem with this relates to point 3. Essentially, these "proofs" of God's existence merely attempt to verify a single aspect that he represents (a creative force, a personification of the human conscience, etc.) and try to show that the represented aspect is a necessity by reducing God to a single principle of reality. From there, by claiming that such a God must exist, they equivocate and suggest that the God of theology, with a variety of unverified traits, desires, and with a supernatural cosmology to attend to, is that God which must exist. This conflation of a single natural principle and a complex, multifaceted intelligent force with specific needs and wants does not cast doubt on God's existence, in of itself. But it does illustrate that the proofs for his existence are generally flawed, and fail to even indicate that something supernatural must exist, despite intentionally mislabeling unknowns "God".
- Religiocentricity- Quite simply, considering the points in 3, and 4, and knowing of all the other religions in the world, it is easy to realize that any given religion is a glorified guess about an essentially unknown and unverifiable aspect of existence. From that perspective, it is hard to accept the dogma pertaining to the speculative deity of that religion's choice, to say nothing about their description about what that deity MUST be like. As such, even if were to concede that a supernatural "god" or something like it were 100% likely to exist, it would be pointless to lump yourself into any given religion, because there are almost infinite possibilities as to the nature of the god(s) in question. It is from there that religion becomes a pointless identification, and it is clear that is focused on a purposefully arbitrary claim.
Well, that's that. One could say that this is actually more consistent with deism than atheism, though. But, since I think that "god" is an arbitrary label, and that it is inappropriate to apply that label to a non-sentient and motiveless creative force or principle, and I also am unconvinced that anything supernatural exists at all, I think "atheism" is a more appropriate description.