An atheist is defined primarily in two senses: Someone who says that he believes there is no God and someone who simply lacks belief in God. An atheist cannot rationally say that he knows there is no God because he would have to know all things in order to know if there is or isn't a God. If he says that he believes there is no God, ask him why he believes that way and begin there. If he says he lacks belief in God, then ask what he does believe in, and start there. I always get around to the question of, "How did we get here?" Since creation and evolution are the only options, I have something further to work with.
An agnostic says that he doesn't know if there is or isn't a God. (Usually after saying this, I challenge them to explain the prophecies of the Old Testament fulfilled in the New. I state how the Bible is unique that way and that only God can make prophecies that are 100% accurate. Then I ask him to explain how that could be done if there is no God).
If there is no God as you say, then in the end I lose nothing. But if there is a God like I say, in the end you lose everything.
Why don't you believe in God? Is there any reason for you to intelligently reject His existence? Or, do you simply desire not to believe in Him?
The Bible doesn't attempt to prove that God exists. It simply speaks as though He does. Maybe I can't prove to you there is a God, but I can introduce Him to you through His Son Jesus Christ, and you can judge for yourself if the Words of Christ in the Bible convince you of His existence.