You gotta stand for something, or you'll fall for anything.
Remembering why we believe what we believe helps us keep actions in line with principles. These are typical atheistic positions on the most common challenges.
1. Humans have a history of making up convenient explanations for things unknown. For example, some had thought the sun to be a living god; others believed (and still do) the earth to be flat and the center of the universe. In short, explanations based on gods and heaven have been steadily replaced as new knowledge is gained. History suggests that remaining mysteries will be sufficiently explained in terms of familiar and rational forces rather than by supernatural ones. For example, many people believe that a superior being created life, while atheists believe that this is just another scientific phenomenon to which people have assigned a supernatural explanation.
2. Explain that 'Intelligent design' is just the awe of new experience or of overwhelming complexity. If a caveman is handed a modern camera, he would not guess that it was a result of step-by-step gains in knowledge over a very long time. It would seem inexplicable, magical, perhaps supernatural. 'The gods must have made it' may be a natural thought, but it would be wrong. Atheists believe that the intelligent design argument is like that of the caveman. Atheists do not believe in the supernatural, so we assign a scientific explanation to all phenomena.
3. Clearly define faith or belief: just because people believe that there is a god, and that they have felt that god personally, does not mean that everyone can do the same. Atheists believe that there are no gods and that gods are not a satisfactory final explanation of real phenomena, because a deity's existence or influence is untestable.
4. Recognize that religions, abstract from their gods, are important as social institutions. People clearly have an innate affinity for stories, akin to the popularity of movies, plays, legends, novels and ballads. Our willingness to believe in well-told stories (even supernatural ones) is a trait of human psychology. Religions influence behavior through scripture and tradition presented with or without explanations. Evolution has selected obedient societies based on religions with beneficial behavior. Self-destructive religions (and their adherents) die out. Atheists believe that the reality of gods plays no part in this explanation (although the reality of the idea of gods does).
5. Clarify that to give up on the gods does not mean you give up on morals. Atheists simply accept responsibility for the moral code by which they live their lives. If you are feeling confrontational, contrast this sharply to the practice of blindly accepting the moral code of an old book.
6. Where did the universe come from? Offering God as an explanation is problematic as it begs the question "Where did God come from?" Atheists admit this may be an unsound argument because it is conceivable that the apparent law of causality may break down at universal scales, just as 'fundamental' laws have been observed to break down at quantum scales. This provides an out for God, in that God needs no independent creator. It also provides an out for the non-existence of God, in that a universe without God would also need no creator. Ultimately, the origin of the universe (or the separation between nature and God) devolves into a question of faith.
7. Point out that Monotheists are themselves atheists with respect to all deities except their own, and that atheists have just taken it one god further. ('We are all atheists about most of the gods societies have ever believed in -- some of us just go one god further.' - R. Dawkins.)
8. Explain that the idea of a personal, eternal, all-powerful God is incoherent. Everything we recognize as personality – the ability to perceive, to have sequences of thought, to have emotions and reactions, etc. – are all things that occur as changes over time. But God is supposed to be changeless and outside of time. So there cannot be a personal, eternal God; if God exists, he is either a finite creature or impersonal. So the God of the Bible, the Tanak, and the Qur'an cannot exist.
9. Try explaining the idea of space and time. According to science, space and time are what we call "The Planes of Existance." According to religious beliefs, God exists outside space and time and if this is the case, he cannot exist.
* Atheists are simply non-religious people. Strained definitions of atheists such as, "they don't believe in God" are fallacious. Which god is the god alluded to within that definition? Atheists are open to the idea that there is a reason why we are here, they just don't accept any human description of that reason.
* People are already atheists to most gods. Christians don't believe in Native American Gods. This is a good argument and cannot be refuted well. It may be more effective to compare the believer's god to an ancient god, like Zeus, who has no followers today.
* Highlight that many religious people afford themselves a questionable sense of comfort by believing in the existence of a non-physical being who watches people, listens to people, communicates with people, and extends help to people. Acknowledge that the fear of letting go of an imaginary friend could affect a person's outlook.
* Many atheists have no beliefs about how the universe came into existence and are comfortable accepting that some things remain unknown, just as religious people do. Many atheists often follow the progress of science, noting that there are falsifiable hypotheses about the origin of the universe with evidence, according to scientists, as opposed to the supernatural hypothesis about the universe with purported evidence, according to religious leaders. Some of those hypotheses suggest that we may be asking the wrong question altogether. Time itself may have begun with the universe, so asking what was there before the universe, might be a nonsensical question, like 'what lies north of the north pole?' or 'what came first, the chicken or the egg?'
* Atheists do not have all the answers. They take the stance that if we don't know, then the issue is a mystery not yet explained scientifically and we should not attempt to create an explanation as religious people do. Most think that accepting the unknown as what it is - unknown - is preferable to unprovable, supernatural explanations of phenomena that historically have been satisfactorily explained without a deity. By shedding religious doctrine, atheists are free to act according to their unconstrained reason, rather than conforming to a set of rules handed down by a religious authority.
* Read up on empiricism and the scientific method, and recognize that an atheism built upon these perspectives can accept the possibility of the 'supernatural', but seeks testable theories and hypotheses to explain events and their causes.
* Employ Occam's Razor, which briefly states that if you are given two explanations for a phenomenon, accept the simpler explanation as true. For instance, many atheists consider the occurrence of simple proteins and evolution into more complex lifeforms a simpler explanation than the existence of a supernatural creator. Consider appeals to undetectable supernatural involvement as inherently not simple and equivalent to resigning the explanation.
* For the most part, atheists tend to accuse anyone of shutting down their opinions. Don't fall into the trap of being most atheists. Remember: Atheists do not have a religion. What does it matter to you if others have a religion? Don't try to convert people, because there really is no reason.
* Some people may, and probably will, disagree with you for dismissing God as an explanation. Sometimes the disagreement may be violent and/or damaging. To avoid this, an atheist can claim to accept God as a possible explanation, and then go on to search for and advance alternate, non-theistic, down-to-earth explanations.
* Occam's Razor can be used perspectively. Use it with caution.