In third world countries such as Brazil, India, Africa etc. religion is very important as from religion they can make 'sense' of life. Religion answers: Who they are? What is their purpose of life? What is life and death? and many other question. In this countries science has not arrived thus religion answers all of these questions.
Other people follow religions (i.e. Buddisum) as they believe in their way of life. By this I mean although they do believe science, big bang, evolution etc. they also want to be vegetarians or do not want to have alcohol or do not want to gamble etc. So religion can be like a border line where the follower must not cross if they do so they will be punished in the going to hell in Christinaty and Jewism or having a bad karma in Hindisum and Buddisum.
But mostly religion brings people together for example, Chrismas or Diwali or any other festivals can be like an excuse where family meet (after long time), have fun and share memories as well as thinking and praying to God. Similarly it bring people in a community together.
I've written this essay about the effects of religion. I plan to post it in the main section. However, in my year or so of lurking here I've noticed that most of the first main posts people create are either irrelevant or bad, and end up down voting the poster to oblivion. To this aim, I first post it here, for your critique, and also to tell me if you think it's appropriate to post in the main section of LessWrong.
The effects of religion
In the atheist community, it's held as pretty much a self-proving truth thar religion is a bad thing. I have attempted to produce a taxonomy of the effects of religion, both positive and negative. This is written based on my personal experience of the Christian church, and on whatever actual facts I could find.
So. My list of the external effects of religion. These are given as both comparisons to normal people, who don't think much about religion or effective charity, and comparisons to LessWrongians.
It's worth pointing out that western society seems to have a lot of cached thoughts from Christianity. Normal people are often not Christian, but casually believe a lot of its teachings. As a result, many of the negative effects of Christianity affect non-Christians who don't pay particular attention to their beliefs too.
The purpose of this essay is to determine if LessWrong should actively evangelize against religion. If we really wanted to, we could probably do so fairly easily. I conclude that it's probably not worthwhile doing so.
The Christians I know all seem to give far more to charity, both in terms of money and time, than average people. Eliezer pointed this out somewhere, but I can't seem to find a reference. The giving probably isn't quite optimized, but it's a far cry better than nothing. A large proportion of the charity which the Christians I know support seems highly effective, and very little of it is optimized for evangelism alone.
It could be that by co-incidence I just happen to know particularly effective Christians for some reason.
It's worth considering the degree to which Christians and atheists disagree on what charities are worth supporting. The only things Christians support which atheists wouldn't as much are things like school chaplains, giving Bibles, and protesting for various Christian issues like opposing gay marriage and abortion. Ridiculous amounts are certainly spent on pointless lobbying, but as a proportion of total Christian charity giving, it can't be that massive.
I don't think there are any atheist organizations which provide enough peer pressure to have the members give as much, and I don't think many people would be as generous over the long term alone as compared to in church groups. So it's an open question to me as to whether your average LessWrongian would do more or less good via charity than your average Christian.
Aside from this, I think many Christians are fairly good at making an effort to be casually kind to those around them: at the very least they aren't as casually cruel as normal people can be. I expect that LessWrongians would be about as good as Christians at this.
Time and money spent on religion
Religious people spend time and money on religious materials, prayer, churches, and so on. The effect of this is probably neutral compared to what a normal person would be doing, as prayer, theology books and such seem to be fairly ineffective but probably not downright negative things. Again, I don't really know what normal people do with their time, but I don't guess that it would be any worse than anything religious. However, this is something which LessWrongians would surely do better at, as they could hopefully spend their time learning useful things or hopefully entertaining themselves in some more meaningful or effective ways.
No cryonics, attitude to death
One of the most important messages I've gotten out of Less Wrong and similar sources is that death is bad. However, religious people disagree, as a result of their belief in an afterlife. I don't know how much this actually matters. Religious people are highly unlikely to sign up for cryonics. However, according to the survey, more LessWrongians are theist than have signed up for cryonics, so I don't think this effect matters much.
For some bizarre reason, all the issues like abortion, the death penalty, bombing civilians in Arab countries, and euthanasia, where belief in a Christian afterlife would seem to me to encourage a left-leaning viewpoint, are also issues where the western church leans to the right. (For example, I'm not quite sure why anyone who believed in hell for nonbelievers would support war against Muslims.) So all these issues where you'd think their religious beliefs would throw them off, it seems more like their conservatism screws up their reasoning. Correlation not causation.
I would also expect most theists to value their own lives, and those of people in their religion, far less highly than those of people with different or no religions. This would be a minor problem, however it doesn't seem to come up at all in the real world.
Practice and or condoning of irrationality
Practicing things like faith, believing in things you have little evidence for, and the above being perceived as a sign of virtue is bad for rationality in other areas of your life. In this section I'm not talking about actual incorrect statements made by the religion. There's nothing in Christianity that explicitly says that, for example, wishing for things and believing you'll get them means that you will. However, Christian thinking implicitly gets your mind used to a world with meaning, sense, and your belief as a determining factor.
In particular, the mind projection fallacy is encouraged by religion. When you believe that there is an omniscient being who controls life, you're encouraged to see patterns where there are none, and see God's character in random events. This is bad.
Also, the central thesis of reductionism, that everything is comprised of ontologically simple elements, is contradicted by religion.
In Christianity at least, you're told that you need sufficient faith in order to successfully pray. This causes lots of rationality based problems. It's one of the axioms of LessWrong-style rationality that what you think does not affect the world. If you're religious, you don't believe that. It leads to things like "believing as hard as you can" and such.
Finally, religion encourages the just world hypothesis, as a result of belief in a benevolent creator. In Christianity, you can always say "But God made it that way" if you support something. This isn't encouraged by the Bible at all, but people still do it.
Actual factual errors in the religion
Obviously, people who believe in a revealed religion are going to walk around wrong about a lot of factual matters. So how many of these actually matter? Things like the power of prayer probably don't matter,as all the Christians I've ever met seem to consume medicine and make health decisions just like the next person.
Believing in creation has a few effects. Firstly, it encourages people to believe that we are well designed. This makes them less likely to accept the idea of cognitive biases. It makes them skeptical of evolutionary psychology, which is bad.
People may get some silly moral ideas, like opposition to homosexuality. But this is decreasing in prevalence, for example as shown by the existence of Christian support for gay marriage and abortion.
Consequentialism is pretty much common sense. However, most religions are phrased in terms of deontology. (This is actually a problem that Christianity doesn't need to have: Paul's comment "Everything is permissible, but not everything I'd beneficial" seems to be as clearly in favor of consequentialism as you're likely to get. Nevertheless, very few Christians seem to get this.) This frequently results in stupid beliefs, like a support of the death penalty, and things such as drug use being "just wrong". However, most normal people seem to default to deontology anyways, so it's hard to say that religion directly causes this problem.
Most Christians would be upset by how frankly LessWrong calls them idiots. As a result, they don't get many of the positive benefits of reading LW type materials. More generally, religious people are going to dislike and mistrust science to a greater extent. There's a lot of benefit to be gained by understanding and trusting science, for example with issues such as climate change.
They're also going to be discouraged from hanging around the intellectual types of people who are otherwise good for you. If you only read Christian media, you're exposed to a far lesser range of media, and you're more susceptible to the general conservative bias which pervades Christianity.
The effect of religious community
Many studies have shown that religiosity correlates with happiness and health. LessWrong seems to have a general consensus that this is as a result of the community created by a religion. Compared to the default position of a normal person, it's way better to be a churchgoer. It remains to be seen if LessWrong groups can be this effective, even though cases such as the New York Less Wrong group seem to be working fairly nicely from what I've heard.
Established religions have an extreme advantage over new organizations such as LessWrong chapters. To start with, they are already large and powerful. There aren't many places where there are enough rationalists to start something like the New York Less Wrong group (notice it's in New York). The people who are drawn to LessWrong are possibly the wrong demographic proportions to create lasting communities, particularly with an excess of young males. It's been previously pointed out that getting girls to show up is essential for LessWrong meetups and communities. So it's hard to get rationalist communities going which can rival religious communities' consistency.
There aren't many organizations like the general Christian church, which provide such a wide ranging base of peer support.
Religion seems to have a variety of positive and negative effects. Its most positive effects are encouraging charity and providing a stable community. The most negative effects are a general mistrust of science, and the various irrationalities which are applauded by religion.
And so, what should a LessWrongian do with respect to religious people? I think we should be polite to them. Religion doesn't have a bad enough effect to justify arguing against it. If by some chance you do convince them out of their faith, the chance that they won't just default to normal person mode and keep the cached thoughts of their religion is fairly low.
Additionally, by arguing with religious people, you make them distrust science and intellectuals and rationality. This is significantly more of a problem than the religion itself. Because of "arguments as soldiers" the religious person might start always looking out for cases of science being wrong, and also never listen to it, because if you listen to science, you're betraying your faith. This is very, very bad, far worse than just compartmentalizing your beliefs.
I recommend a policy of "raising the sanity waterline". Just casually improving everyone's rationality would be a far more effective goal. It doesn't look like being religious significantly affects your mental abilities in other fields: look at the proportion of religious Nobel prize winners.
There's little upside in specifically attempting to evangelize theists, so I suggest we shouldn't.