The question that seems most key to the religious debate, and which is often asked is "Do you believe in the existence of God?".
Even the question itself reveals much about the cultural basis of our beliefs, because it is only in certain cultures that a single god is the prevailing belief system. A person genuinely enquiring into the spiritual truths of the universe free from cultural constraints might ask "Are their any other powers in the universe who's existence is not apparent to us?". Instead by asking whether "God" exists, we show the way in which our culture often defines the range of beliefs from which we select a portion, which is not real freedom of choice or thought. The dominant religious concept is of a single "God", therefore we ask if "God" exists, rather than whether gods exist, mountains have spirits, or our dead ancestors are controlling our lives as one would in other cultures with different belief systems.
This is just one way in which the cultural basis of religious belief is obvious, there are no religions that have manifested themselves in different cultures in the same form, with no connection between them. If they related to some central truth, one would expect that they would not be propagated like other social customs, person to person, but would instead appear as spontaneous manifestations of a trans-cultural truth independently. Where the connections between people ended in a world before mass travel and communication, so did one eternal and unchanging truth end and a new completely differing one begin.
Historically the predominant mechanism by which the religions have spread has been the sword. Only by the might of the Roman empire did Christianity become the religion of Europeans, the armies of Islam converted the Middle East, the ruthless Emperor Ashoka converted to Buddhism and bade the rest of his empire to follow. Conquering armies and the edicts of the powerful have forged the path for the major religions through our world. Though religion has generally been found wanting as a civilizing force, it has been highly effective as a means of control, for unifying people to maintain kingdoms and conquer others lands. Many followers of the major religions profess that they are religions of peace, which requires ignorance of their history and a selective interpretation of their theory.
Returning to the question of the existence of some power beyond our general knowledge, should there be a rational basis for belief in such a being?
Many people in our society think our inability to understand the complexity of the universe as evidence for the existence of a higher power that does understand and is in control. This is the same basis for the belief that the sun was a god, or that lightning was some indication of a gods anger, beliefs that less scientifically mature cultures still hold. At their base, they are all attempts to explain the phenomena of the universe around us which we don't understand. It is a reversal of logic, our lack of understanding should make us cautious and humble about committing to grand explanations like unprovable, omnipotent gods, rather than lead us to accept such explanations. It is telling that the level of religious belief generally corresponds inversely to the level of education in a society, gods seem to shy away from the knowledgeable. The reason that we see the belief in a sun god as primitive, and the belief in the whole cosmos as an omniscient, omnipresent god as rational, is purely culturally based.
The proliferation of religions around the world, shows there is a deep need for a certain sort of understanding in the human psyche, that religions fulfil. The reason for the propagation of religions is probably evolutionary like much that passes through our culture unquestioned, a child need not know all the reasons behind it's parents aversion to a certain poisonous plant, as the parents themselves may not, it just needs to avoid it or death could be the penalty for inquisitiveness. So it is that the vast majority of people hold to core beliefs of their parents, and don't question them, so on down through the ages. The percentage of religious people who follow the approximate beliefs of their parents is almost total. Even Mohammed, the Buddha and Jesus only modified versions of the religions of the society around them, showing their own cultural inheritance and avoiding the need for potential converts to accept totally new concepts no doubt played a large part in the success of their religious ideas.
The philosopher Pascal wagered that one must choose in this life whether or not to accept the existence of god, and that if one chose unbelief when a judgemental god indeed existed the consequence would be an eternity of suffering in "hell", whereas the price of belief in the afterlife if there is no such god is nothing, so therefore a rational person would choose belief. Again the cultural basis of belief comes in, as Pascal's Christianity is not the only faith in the world or no doubt even his country, so the wager was never as simple as he believed. Assuming there is only one true faith among the countless faiths and variations between them, statistically speaking, Pascal's faith was almost certainly a false one. The first of the commandments of the god Pascal accepted was "Thou shalt have no other gods before me", if other gods were similarly jealous how might the wrong choice play out in the afterlife.
This is one of the greatest arguments against following any specific religion, that the evidence supporting any one religion is no better than that of any other religion. For every person healed, every miracle performed, every person who claims to know a god personally, there are any number of similar examples of divinity from other fundamentally different and contradictory faiths. Over time their have been thousands of religions with vastly different and directly opposed commandments and rules. To choose one religion, one must generally deny the truth of all others. Each disagrees with the other and often believe that your choice of a different religion consigns you to their version of hell.
Some of the worst religious conflict is between branches of religions scarcely distinguishable from one another to an outsider. Therefore if such subtleties can consign one to eternal suffering and damnation, considering the consequences one would do well to have unquestionable evidence for denying every other religion but one. The fact that many religions specifically deny that one should use reason for believing in them means one is not expected to have any sort of rational basis for the choice. If you consider yourself a rational person, then other than having some supernatural experience that could not possibly otherwise be explained, it is difficult how one could intelligently believe with absolute certainty in one of the worlds millions of gods, thousands of religions, and more specifically one of the many sects of each individual religion, to the exclusion of all others. An irrational person can simply choose and ignore inconvenient facts, but this is not a path open to us who would have truth and reason as part of the governing forces in our lives.
Fantastical claims require fantastic evidence, and the most fantastical claim of all, the existence of all powerful gods that control the entire universe, is found wanting.
As a side note most religions deny the use of intelligence other than to follow their particular commands anyway, Bertrand Russell said he could not find one word of praise for intelligence anywhere in the bible, and other religions are similarly afflicted if not adamantly against independent thought. This should be unsettling to a person who understands what effects blind adherence to power has had on the world. Voltaire said it best "Those who can make you believe absurdities can make you commit atrocities". A belief in a religion becomes the highest morality in someone's life, therefore if they can be convinced that the gods really want some action to be performed, regardless of the morality of the act, the gods will, is the higher morality. When these are mythical beings, the belief in which is used for the purposes of control by humans seeking power, the consequences have been unthinkable.
People often don't have particularly good reasons for their religious beliefs. The vague "belief in something greater than us" is given as the basis for many peoples belief in their particular religion. This is not a rational basis for following any specific religion, but added to cultural influences it incorrectly becomes one for many. A vague feeling might be a reason for not discounting the possibility of some sort of higher powers existence, it is not a valid reason for moulding your entire life around an arbitrary way of living. It is a simple logical error, that the belief that gods exist, by however many people, is in any way evidence that they do exist. As Gandhi would have agreed, a statement does not become true by repetition, though he may have agreed with little else that I have said here.
It would seem one of the bases for the prevalence of religion in our world is the inability to tell the difference between ones beliefs and ones knowledge. A vague feeling is not knowledge. In my experience no-one really knows whether the gods exist or not, they only have the belief they do. Those who can't tell the difference between belief and knowledge, a simple logical error, are fertile ground for religion, superstition, and as history has shown, far worse.
People often follow a religion because they like certain aspects of it, which might conform to their beliefs. Whether one likes a religion, agrees with it, thinks it would create a better world etc, is irrelevant. Ultimate truth is not the product of our likes and dislikes, truth can only be discovered by rational enquiry, not bought into being by the superstitions or whims of humanity. One cannot simply choose a religion and therefore ultimate truth, because one cannot choose the composition of the universe they exist within. A higher power either exists or it doesn't, belief in it one way or the other will not make it truth.
The brutality displayed by people claiming to follow one religion or another reflects the potential for brutality within existence generally. For a person to convince me to follow a god, they would not only have to prove the unlikely fact of its existence, but explain why, in a world with so much unjust suffering, the gods mere existence make them worthy to fashion my entire life around their whim (which generally seem to involve a lot of rambling about how great and magnificent they are). Indeed looking at much of the current world, it would seem more logical to set oneself against the gods, as it would seem in the case of those who claim omnipotence, the happiness of humanity is of little concern to them. Luckily the reliable evidence that gods exist is non existent, therefore our opposition in changing the world is neither omniscient nor omnipotent.
Studies have shown some psychological benefits from holding a belief in one religion or another, different religions seek to answer the difficult questions of each age by placing them in the hands of unseen powers. Whilst religion may be a placebo for the difficulties of higher consciousness, it bodes ill for the intellectual progression of our culture if we seek refuge in delusions.
Often for those who would change the irrational religious beliefs of mankind, the proposition that religion is a delusion is not expected to be replaced by anything. Many would take an entire belief system away from people, and leave them intellectually undefended against the incomprehensibility of existence. We need a more comprehensive perceived answer to religion than nihilism.
The fact that the belief in a religion can be shown to have positive psychological benefits shows the need for people to have something to believe in, to know that are part of a greater scheme.
Whilst consumerism has become the hierarchical basis and spiritual refuge for many people, it does little to satisfy those who are unconsciously running from the perceived inherent meaninglessness and pointlessness in the universe. Predictably surrounding ourselves with objects does not fulfil us. Whilst people get temporary relief in consuming, as the rising rates of depression in many capitalist societies show, it is not lasting. This fleeting nature strengthens consumerism in the short term, people obsessively return to it for it's elusive promise of contentment, but it is an empty promise. Studies show that since the second world war our society is on average twice as wealthy but no happier.
Similarly with nationalism and other "ism's" that people try to sell as a meaning for our lives, they are mechanisms of control just as religion is. Nationalism tries to tell us we are set apart from others just because of the random chance of our birth, thereby trying to offer us happiness by grounding our self esteem in superiority over others. It is a modern version of religion, used for control, used to define us and them, to goad powerless people into passion for war and competition. We have seen the self destruction of many of these movements over the last century, though they offer the intoxication of short term mindlessness, their negative basis inevitably leads to hatred and anger that destroys them from within.
So what would offer us the happiness that we seek, that has a positive base in the reality of our lives?
Short term happiness should be replaced by healthy lifestyles; positive thinking, nourishing, enjoyable food, dependable, close relationships, caring sexuality, a compassionate society, humour, the appreciation of beauty, exercise etc. Our society should be based on increasing the number of healthy, positive experiences in our lives, understanding that our happiness is a best assured as a by product of a caring society.
The longer term, lasting happiness is based on a positive, healthy lifestyle above, but it also needs to provide a rational basis for our place in the cosmos, and for our lives to have meaning. For this understanding I believe in many ways we need to follow what we seek to replace, religion, but replace its ancient crafted delusions with modern thinking that can make a positive difference in the way we treat the world and each other.
If there is no higher consciousness to tell us what meaning is, then why should it follow that reality is a meaningless void? If there are no gods, if in their place we are the highest consciousness in the universe, then it is up to us to define a meaning for ourselves. If we seek to live in a more just, humane world, as the highest creative power in the universe, we should take on the morality of the creator we would wish to have. Some would say this makes meaning an opinion, however the basis on which I would choose to construct meaning, pain, pleasure, freedom, happiness, are evolutionary shared realities which transcend the potentially arbitrary and selfish nature of opinion.
There may be no inherent morality in evolutionary logic, but morality has come into being within ourselves. In our actions we have the ability to make individual choices, and we can use those choices to create pieces of existence which are compassionate and just. We can be the gods we would wish to have. All is not within our control, we cannot change the laws of the whole universe, we can not necessarily change everything that happens to us, but we can change the part of existence which our own actions touch.
Ultimately the universe is not a moral void, if we are not.