Many, if not most atheists were previously religious. As has been explained above, the vast majority have seriously considered the possibility that God exists. Many atheists have spent time in prayer trying to reach God.
Of course, it is true that some atheists lack an open mind; but assuming that all atheists are biased and insincere is offensive and closed-minded. Comments such as "Of course God is there, you just aren't looking properly" are likely to be viewed as patronizing.
Certainly, if you wish to engage in philosophical debate with atheists it is vital that you give them the benefit of the doubt and assume that they are being sincere if they say that they have searched for God. If you are not willing to believe that they are basically telling the truth, debate is futile.
"Isn't the whole of life completely pointless to an atheist?"
Perhaps it is to some, but still, many atheists live a purposeful life. They decide what they think gives meaning to life, and they pursue those goals. They try to make their lives count, not by wishing for eternal life, but by having an influence on other people who will live on. For example, an atheist may dedicate his life to political reform, in the hope of leaving his mark on history.
It is a natural human tendency to look for "meaning" or "purpose" in random events. However, it is by no means obvious that "life" is the sort of thing that has a "meaning".
To put it another way, not everything which looks like a question is actually a sensible thing to ask. Some atheists believe that asking "What is the meaning of life?" is as silly as asking "What is the meaning of a cup of coffee?". They believe that life has no purpose or meaning, it just is.
Also, if some sort of mystical external force is required to give one's existence a "meaning", surely that makes any hypothetical god's existence meaningless?
"So how do atheists find comfort in time of danger?"
There are many ways of obtaining comfort:
Your family and friends
Food and drink
Music, television, literature, arts and entertainment
Sports or exercise
That may sound like rather an empty and vulnerable way to face danger, but so what? Should individuals believe in things because they are comforting, or should they face reality no matter how harsh it might be?
In the end, it's a decision for the individual concerned. Most atheists are unable to believe something they would not otherwise believe merely because it makes them feel comfortable. They put truth before comfort, and consider that if searching for truth sometimes makes them feel unhappy, that's just hard luck. Often truth hurts.
"Don't atheists worry that they might suddenly be shown to be wrong?"
The short answer is "No, do you?"
Many atheists have been atheists for years. They have encountered many arguments and much supposed evidence for the existence of God, but they have found all of it to be invalid or inconclusive.
Thousands of years of religious belief haven't resulted in any good proof of the existence of God. Atheists therefore tend to feel that they are unlikely to be proved wrong in the immediate future, and they stop worrying about it.
"So why should theists question their beliefs? Don't the same arguments apply?"
No, because the beliefs being questioned are not similar. Weak atheism is the sceptical "default position" to take; it asserts nothing. Strong atheism is a negative belief. Theism is a very strong positive belief.
Atheists sometimes also argue that theists should question their beliefs because of the very real harm they can cause -- not just to the believers, but to everyone else.
"What sort of harm?"
Religion represents a huge financial and work burden on mankind. It's not just a matter of religious believers wasting their money on church buildings; think of all the time and effort spent building churches, praying, and so on. Imagine how that effort could be better spent.
Many theists believe in miracle healing. There have been plenty of instances of ill people being "healed" by a priest, ceasing to take the medicines prescribed to them by doctors, and dying as a result. Some theists have died because they have refused blood transfusions on religious grounds.
It is arguable that the Catholic Church's opposition to birth control -- and condoms in particular -- is increasing the problem of overpopulation in many third-world countries and contributing to the spread of AIDS world-wide.
Religious believers have been known to murder their children rather than allow their children to become atheists or marry someone of a different religion. Religious leaders have been known to justify murder on the grounds of blasphemy.
There have been many religious wars. Even if we accept the argument that religion was not the true cause of those wars, it was still used as an effective justification for them.
"Those weren't real believers. They just claimed to be believers as some sort of excuse."
This is rather like the No True Scotsman fallacy.
What makes a real believer? There are so many One True Religions it's hard to tell. Look at Christianity: there are many competing groups, all convinced that they are the only true Christians. Sometimes they even fight and kill each other. How is an atheist supposed to decide who's a real Christian and who isn't, when even the major Christian churches like the Catholic Church and the Church of England can't decide amongst themselves?
In the end, most atheists take a pragmatic view, and decide that anyone who calls himself a Christian, and uses Christian belief or dogma to justify his actions, should be considered a Christian. Maybe some of those Christians are just perverting Christian teaching for their own ends -- but surely if the Bible can be so readily used to support un-Christian acts it can't be much of a moral code? If the Bible is the word of God, why couldn't he have made it less easy to misinterpret? And how do you know that your beliefs aren't a perversion of what your God intended?
If there is no single unambiguous interpretation of the Bible, then why should an atheist take one interpretation over another just on your say-so? Sorry, but if someone claims that he believes in Jesus and that he murdered others because Jesus and the Bible told him to do so, we must call him a Christian.
"Obviously those extreme sorts of beliefs should be questioned. But since nobody has ever proved that God does not exist, it must be very unlikely that more basic religious beliefs, shared by all faiths, are nonsense."
The commonality of many basic religious beliefs is hardly surprising, if you take the view that religion is a product of society. From that viewpoint, religions have borrowed ideas which contribute to a stable society -- such as respect for authority figures, a prohibition against murder, and so on.
In addition, many common religious themes have been passed on to later religions. For example, it has been suggested that the Ten Commandments of the Old Testament actually have their roots in Hamurabi's code.
The claim that because something hasn't been proved false, it's less likely to be nonsense, does not hold. As was pointed out earlier in this dialogue, positive assertions concerning the existence of entities are inherently much harder to disprove than negative ones. Nobody has ever proved that unicorns don't exist, and there are many stories about them, but that doesn't make it unlikely that they are myths.
It is therefore much more valid to hold a negative assertion by default than it is to hold a positive assertion by default. Of course, "weak" atheists may argue that asserting nothing is better still.
"Well, if atheism's so great, why are there so many theists?"
Unfortunately, the popularity of a belief has little to do with how "correct" it is, or whether it "works"; consider how many people believe in astrology, graphology, and other pseudo-sciences.
Many atheists feel that it is simply a human weakness to want to believe in gods. Certainly in many primitive human societies, religion allows the people to deal with phenomena that they do not adequately understand.
Of course, there's more to religion than that. In the industrialized world, we find people believing in religious explanations of phenomena even when there are perfectly adequate natural explanations. Religion may have started as a means of attempting to explain the world, but nowadays it serves other purposes as well. For instance, for many people religion fulfils a social function, providing a sense of community and belonging.
"But so many cultures have developed religions. Surely that must say something?"
Not really. Most religions are only superficially similar; for example, it's worth remembering that religions such as Buddhism and Taoism lack any sort of concept of God in the Christian sense. In short, there is no consensus amongst religions as to what God actually is. Hence one of the problems you must face if you wish to discuss God with an atheist, is that of defining exactly what you mean by the word.
Also, most religions are quick to denounce competing religions, so it's rather odd to use one religion to try and justify another.
"What about all the famous scientists and philosophers who have concluded that God exists?"
Firstly, note that surveys typically find that around 40% of scientists believe in god; so believers are in the minority. (The most recent survey was by Edward J. Larson and Larry Witham, was carried out in 1996, and was reported in the journal "Nature".)
To be continued...