A popular argument for atheism - along with being a popular "scientific" rebuttal of Intelligent Design - is the notion of the "infinite regress". Every action or being requires a cause, so if God exists, God must have a cause; that cause also had a cause, and so on.
Christian theology (and maybe some other religions as well) states that God is the "uncaused First Cause" or that He is "eternally self-existent". I've even heard Him described as being "beyond time and space".
It's not easy to choose between "no intelligent cause" and an uncreated God. (Some days, I simply flip a coin. ;-) --Ed Poor Talk 15:35, 8 April 2008 (EDT)