A comment that I often hear is that atheists are absolutely certain that a god does not exist, when this is obviously unknowable. I would never say that I am absolutely certain that a god does not exist because I may be wrong. But this question is loaded, because it forces atheists to admit that there is a possibility that a god exists and that - in the minds of some theists - opens up the argument to a 'what if you're wrong?' style of rebuttal. I think that the first step is to define god.
So how do I define god? Obviously there are many different attributes and definitions we have granted to the idea of a deity. But which ones do I mean when I say that I do not believe in god? First of all, I generally mean the god of the Bible, the god of the Torah, the god of the Koran. I don't believe that any of those characters exist in any real sense. That I believe flat out, and with very little reservation. I feel confident stating that I am absolutely certain that god - as represented in the Bible - does not exist. The character in the Bible is not consistent with what we know of the world, what we know of nature and science and history. Now if a being named Yahweh does indeed exist, and if he truly expects obedience and worship at the threat of punishment and eternal damnation then he is either wicked or incompetent, for he has not properly revealed himself to humanity. That is my first answer.
But what of the beginning of time? How did the universe get started? And even if we accept evolution as an explanation for the ascension of life, is it inconceivable that it was prompted and maybe even nudged along by a powerful, or even an omnipotent deity? No, that is not inconceivable. But if it happened, then that deity has gone to great lengths to hid his or her or its tracks and clearly that deity should not expect any kind of worship or devotion from humanity. In this case, we have essentially defined a deity that is synonymous with 'nature' and since we already have a concept of nature, then there is no need to confuse the issue.
How about intangible things, such as love and wonder and longing? It may be possible that those are simply neurological functions, or perhaps even side effects of more basic brain functions. But what if our deity is outside of nature and only manifests itself in these ways? Well then god is love and again, we already have a definition of that.
At the end of the day, anyone can define god in any way they wish. It only matters when that god demands things of them. If no demands are made, then a personal deity is little more than a guess and a romanticization of a preexisting idea.