Religion is a type of irrational false belief that people cling to despite total lack of evidence for it, thus perpetuating thousands of years of murder, dogma, oppression, and rejection of science.
Religion and insanity
Common religious delusions include belief in supernatural beings and belief in good or bad magical lands people go when they die . There is no known concrete, verifiable evidence supporting the claims of any religion. Religion is a delusion and is as irrational as many beliefs that people in psychiatric hospitals hold. Despite this people who in other areas think and act rationally and are not considered psychiatrically ill sometimes hold outrageously irrational religious beliefs. 
According to Marxist theory religion is the opium of the people, designed to keep conservatives in power and to prevent the poor masses from improving their only lives. Religion has been used historically to justify the power of royalty and nobility etc. Even today the Religious Right in the United States use Christianity to try and justify Rich Americans holding onto wealth and power.
Religion was and is used the way Marx theorised to keep wealthy oppressors in power but that's not the whole story by any means. Religion is also found in many egalitarian hunter gatherer societies where there is no oppression or exploitation and some religious traditions like Liberation theology encourage people to fight oppressors.
Thousands of years ago, when humans were still hunters and gatherers, they were very curious as to what is everything around them. After evolving to have such a large brain capacity, human beings soon started realizing and questioning everything. They started thinking: why are we here, why does it rain, why do I feel things, why does it get hot, why does it get cold, where do we come from, what are our origins, why do we exist, why does anything exist, what is that big, hot, shiny thing in the sky, etc... Humanity didn't yet have science to figure things out, so they came up with simple-minded answers like god and gods. Basically, religion is when humans thinks of what their Earthly origin is, without the knowledge that comes from scientific evidence. A perfect example of this is when the Spanish conquistadors invaded South America: the natives didn't understand the technology of the gunpowder that the Spanish used, so they thought that the Spaniards were gods.
Religion nowadays persists despite good scientific explanations for the way the world is. Believers just don't know or they are too weak to break free from Childhood indoctrination and think rationally. The human brain processes positive information better than negative information. If it were the other way around, human beings would easily fall into depression and mass suicides would happen. Their religions have been passed down from thousands of generations and to find out what they believe in is false, is quite a rude awakening. Thankfully though, there are open-minded humans who can take the truth and accept that we have a limited time on this planet: let's make the most of it, instead of trying to prepare for nothing.
An atheist's philosophy- the object in life is to enjoy it as much as you can and make it last as long as possible. Also try and make the world a better place so good things you've started can continue after you die. There are also pseudo-religions that help to further awareness of the irrationality of religion (see Church of the Flying Spaghetti Monster)
There is no evidence for God or gods. Further different religions contradict each other and they can't all be true. There is no rational reason to prefer one religion over another. The supporters of the different religions keep telling us, "We're right! All the others are wrong. Please believe what we say." There really is no reason to believe any of them.
Religion is rather like a severe form of schizophrenia, as people believe that a non-existent something is always by their side. Many people around the world constantly try to converse with an imaginary, abusive being who will murder anyone who disagrees with him. The imaginary being commonly known as "God" is likely a male, as religion is often misogynistic. God no more likely to exist than the Tooth fairy, Santa Claus or the Easter bunny.
According to Marxism religion is the opium of the people. There is an element of truth in the argument that religion tries to give purpose and meaning to suffering. Then in the worst cases people get religious about suffering and don't do enough to end it. Capitalism doesn't cause religion. Pre-capitalist societies are also usually religious. Capitalist societies all too often use religion to persuade exploited people to put up with whatever is wrong. Religion is one of the causes of suffering in the modern world.
Religion and Liberalism
Some liberals have no religion. Other liberals interpret their religion in ways that support liberal philosophy. American conservatives from the Religious Right love dogmatic fundamentalist Christianity because conservatism substitutes standard thoughts and behaviors for rational thought, ethical and spiritual growth. Conservatives worldwide tend to love whatever religion is traditional in their community. There are a few conservative atheists as well. Some more moderate believers go to church but they are not so sure about who or what God actually is. This is true of many Unitarians.
Do not confuse religion with a spiritual search of any kind.
Many of the things that religion did and does to mankind are awful. Religion also did some good things (it’s hard to give proportions).
- Right here:
- Faith or delusion? At the crossroads of religion and psychosis.
- Contribution to the Critique of Hegel's Philosophy of Right (1843), Karl Marx, "It is the opium of the people."