We all have various reasons for the choices we make. Usually we will have more than one reason for making a particular choice, even if it is something simple and inconsequential such as choosing our favorite style of shoes. When the choice is something important, such as choosing what to believe about religion and the reason for our existence, the chances are good that we have more than one reason for that choice. The chances are also good that we have reasons that speak to us very strongly on an emotional level. We probably also have reasons that are more purely logical.
Whenever discussing one's atheism, whether with religious people, agnostics, or even other atheists, it is good to keep in mind this distinction between the emotional reasons that give us the strength to resist the pressure to conform and the logical reasons that show we are right.
The religious are all very eager to focus on our emotional reasons, because they know such reasons are insufficient (even though they use such reasons themselves). They know perfectly well that they have chosen to believe for reasons that are not logical and they want to pretend that non-believers have done the same. (Hence the frequently heard canard that atheism is "just another religion".)
The religious know they can't win the argument based on logic. They will try to convince you that their reasoning is logical, but, when they fail, they fall back on the accusation that the non-believer's reasoning is "just as illogical".
Don't play into their hands by citing your emotional grounds as your primary or logical grounds.
For example, a frequently mentioned emotional ground for disbelief is the level of evil and suffering one sees in the world. This
evil and suffering, however, do not prove that god doesn't exist. After all, he could just be a sadistic bastard, or, as the religious maintain, have inscrutable reasons for causing us--his supposed favored creations--to live in a world of evil and suffering.
The evil in the world may well have been an important motivating factor that led many people to atheism but it is hardly conclusive. At best, it provides a partial refutation of assertions that a benevolent god exists. It is a sufficient reason only on an emotional level.
In this context, it is important to keep in mind that believers truly believe the things they say. They truly believe that this life is but a brief and relatively unimportant interlude before an eternity of bliss. To someone who believes this sort of thing, it is very easy to rationalize away the problem of evil and suffering in the world. Those things are seen as some sort of test or training before humans reach their infinite afterlife.
Likewise,
don't mention personal misfortunes and god's failure to answer your prayers back when you were a believer. It may be that the death of a loved one or some other past catastrophe for which you fervently asked god's intervention was a crucial turning point for you personally. You must remember that was a turning point for you only because it allowed you to throw off the bias instilled in you by childhood brainwashing and finally realize that those studies proving the inefficacy of prayer were right--not because that one example proved the point.
Mention the studies, certainly, but if you mention your personal disappointment, the believer will conclude that you are "
just mad at god". In fact, I suspect that this particular response from believers comes from their experiences with non-believers who haven't presented their non-belief in a logical fashion.
Most important of all,
try to avoid mentioning your disagreement with religious morality. One of the most pernicious and damaging canards that the religious use to justify their bigotry toward non-believers is the notion that we reject god because we don't want to lead moral lives. This prejudice is so pervasive and ingrained in the religious that I would suggest that you avoid mentioning your disagreements on such matters to the religious at all--even if you are not discussing god's existence.
If this point does come up, as it almost always does, it is good to point out that study after study shows that religious people violate their religious morality as often (usually more often) as non-believers do. After that, explain how
religious morality fails as a moral code and is really just fear of punishment, authoritarianism, and conformity.
When believers ask you why you don't believe simply
make the logical case--point to the lack of evidence for god, the evidence that gods are man-made, and your unwillingness to
dishonestly apply different standards to different supernatural propositions.
If any of your emotional motivations come up in conversation, just make clear that they are no more than that. You can say something like:
"
That is why it doesn't bother me emotionally that I don't believe in god."