Probably the most common atheist argument is that there is no evidence that God exists. However, it is unclear what would constitute positive empirical evidence for God's existence. Most theological arguments for God's existence are rational rather than empirical - except perhaps for the argument for design, based on the inherent rationality evidenced by the material world. How can human beings assess the evidence for an infinite being that is, by definition beyond their comprehension? What would a world without God look like, compared to a world with God - and how can we be sure enough about this to make the comparison? In many ways, the concept of God is beyond the material world, and as such empirical evidence cannot legitimately have any bearing on whether or not we accept it.
This also brings the question of what it would mean to believe in God at all. If God is truly infinite and beyond human comprehension, then is whatever believers have in mind when they think about God, really God? If an image, or feeling, or idea comes to mind when thinking about God, then this limited idea can never really be God, who is a totally infinite being. Then, our use of the word "God" itself becomes an empty signifier, a vague attribution of sapience to the universe without any concrete application.