No one is born an atheist. People choose to become atheists as much as they choose to become Christians. And no matter how strenuously some may try to deny it, atheism is a belief system. It requires faith that God does not exist.
When dialoguing with atheists, it is helpful to point out the logical problems inherent in their belief system. If you succeed in showing an atheist the natural outcome of some of his (or her) main claims and arguments, you are in a much better position to share the gospel with him. Let us consider two prime examples here.
(1) "There is no God." Some atheists categorically state that there is no God, and all atheists, by definition, believe it. And yet, this assertion is logically indefensible. A person would have to be omniscient and omnipresent to be able to say from his own pool of knowledge that there is no God. Only someone who is capable of being in all places at the same time--with a perfect knowledge of all that is in the universe--can make such a statement based on the facts. To put it another way, a person would have to be God in order to say there is no God.
This point can be forcefully emphasized by asking the atheist if he has ever visited the Library of Congress in Washington D.C. Mention that the library presently contains over 70 million items (books, magazines, journals, etc.). Also point out that hundreds of thousands of these were written by scholars and specialists in the various academic fields. Then ask the following question: "What percentage of the collective knowledge recorded in the volumes in this library would you say are within your own pool of knowledge and experience?" The atheist will likely respond, "I don't know. I guess a fraction of one percent." You can then ask: "Do you think it is logically possible that God may exist in the 99.9 percent that is outside your pool of knowledge and experience?" Even if the atheist refuses to admit the possibility, you have made your point and he knows it.
(2) "I don't believe in God because there is so much evil in the world." Many atheists consider the problem of evil an airtight proof that God does not exist. They often say something like: "I know there is no God because if He existed, He never would have let Hitler murder six million Jews."
A good approach to an argument like this is to say something to this effect: "Since you brought up this issue, the burden lies on you to prove that evil actually exists in the world. So let me ask you: by what criteria do you judge some things to be evil and other things not to be evil? By what process do you distinguish evil from good?" The atheist may hedge and say: "I just know that some things are evil. It's obvious." Don't accept such an evasive answer. Insist that he tell you how he knows that some things are evil. He must be forced to face the illogical foundation of his belief system.
After he struggles with this a few moments, point out to him that it is impossible to distinguish evil from good unless one has an infinite reference point which is absolutely good. Otherwise one is like a boat at sea on a cloudy night without a compass (i.e., there would be no way to distinguish north from south without the absolute reference point of the compass needle).
The infinite reference point for distinguishing good from evil can only be found in the person of God, for God alone can exhaust the definition of "absolutely good." If God does not exist, then there are no moral absolutes by which one has the right to judge something (or someone) as being evil. More specifically, if God does not exist, there is no ultimate basis to judge the crimes of Hitler. Seen in this light, the reality of evil actually requires the existence of God, rather than disproving it.
At this point, the atheist may raise the objection that if God does in fact exist, then why hasn't He dealt with the problem of evil in the world. You can disarm this objection by pointing out that God is dealing with the problem of evil, but in a progressive way. The false assumption on the part of the atheist is that God's only choice is to deal with evil all at once in a single act. God, however, is dealing with the problem of evil throughout all human history. One day in the future, Christ will return, strip power away from the wicked, and hold all men and women accountable for the things they did during their time on earth. Justice will ultimately prevail. Those who enter eternity without having trusted in Christ for salvation will understand just how effectively God has dealt with the problem of evil.
If the atheist responds that it shouldn't take all of human history for an omnipotent God to solve the problem of evil, you might respond by saying: "Ok. Let's do it your way. Hypothetically speaking, let's say that at this very moment, God declared that all evil in the world will now simply cease to exist. Every human being on the planet--present company included--would simply vanish into oblivion. Would this solution be preferable to you?"
The atheist may argue that a better solution must surely be available. He may even suggest that God could have created man in such a way that man would never sin, thus avoiding evil altogether. This idea can be countered by pointing out that such a scenario would mean that man is no longer man. He would no longer have the capacity to make choices. This scenario would require that God create robots who act only in programmed ways.
If the atheist persists and says there must be a better solution to the problem of evil, suggest a simple test. Give him about five minutes to formulate a solution to the problem of evil that (1) does not destroy human freedom, or (2) cause God to violate His nature (e.g., His attributes of absolute holiness, justice, and mercy) in some way. After five minutes, ask him what he came up with. Don't expect much of an answer.
Your goal, of course, is not simply to tear down the atheist's belief system. After demonstrating some of the logical impossibilities of his claims, share with him some of the logical evidence for redemption in Jesus Christ, and the infinite benefits that it brings. Perhaps through your witness and prayers his faith in atheism will be overturned by a newfound faith in Christ.