Well, of course, here's another subject that most of us have strong feelings about, one way or another. I realize it's a dicey issue for me to wade into this one as a psychiatrist.
Here are my frank opinions:
There are many varieties of religious belief and practice. Many religions hold views that are quite opposite or contradictory to what other religions hold. Even subgroups of the same religious group can have vigorous differences in belief.
As far as the literal beliefs themselves go, I as a scientist would be closest in my view to Richard Dawkins, who boldly pronounces a belief in God to be a "delusion."
Yet I think there is a lot of wisdom, beauty, and truth that Dawkins misses with his pronouncement (accurate though it may be on one level) of delusiveness. Here, I think someone like Joseph Campbell is a wiser figure, in that his analysis of religious belief is anthropological, where religious stories are understood as metaphors, often with pearls of wisdom or insights about the human condition. His view is that most every religion or mythological system shares similar stories and insights.
This is my own view -- religious stories contain metaphorical insights and truths, which can teach, guide, warn, or comfort us. In a sense these insights and truths could be understood as part of what "God" is (literally). Interesting phrases such as "the Word made flesh" found in the Bible (John 1:14) exemplify the concept that the application or vivification of ideas or metaphors can be understood as the core of what "God" is.
Mind you, many religious stories may be very much coloured by the impressions or cultural values of the human authors of the stories--and perhaps of the subsequent editors over the years or centuries. Some of these values may in some cases be quite idiosyncratic or highly influenced by the conservatism or liberalism of the individuals in their time, and not very reflective of deep truths about humanity or life.
In any case, I do think that many religious stories contain valuable insights, also their familiarity to people over many centuries or even millenia, have caused the stories to become more richly ingrained in the culture. Perhaps the passage of time, of many generations, acts as a sort of "cultural filter" through which religious texts pass, allowing the texts to acquire more universal relevance. I encourage the interested reader to look at some of Northrop Frye's work: he was an important scholar who looked at the deep impact of religious texts on literature, with the keen eye of a literary critic.
Some of the richer psychiatric theory of the past century looks deeply at the human condition, at unconscious drives & motivations; art, literature, and religious mythology are important illuminations in this psychological exploration. The creative act of participating in the arts, literature, and in religious metaphor, can be a healing act--from a psychiatric point of view, or in a broader way-- provided the experience is not simply part of a neurotic defense (e.g. projection, suppression, distraction, rationalization, denial, etc.).
There are several advantages to "organized religion":
1) there is a community of people who care about each other, who care about the community itself, and hopefully who care about other external communities. This is healthy, and there are not enough such communities outside of religion in our culture today. So organized religion can be a source of friendship, social warmth, an antidote to loneliness.
2) Also, most religious groups are devoted to altruistic service in some way; there is absolutely no doubt that altruistic service is psychologically healthy. It can be hard to find satisfying altruistic opportunities outside of such an organized setting
3) Many religious groups celebrate a long cultural heritage of its members; this can add to a sense of meaningfulness and a connection to the past, and to family. Unfortunately, many individuals may feel excluded by, or that they do not fit in with, the group's cultural heritage. I think it is important to find a group that suits your own personal culture, and I think there are more choices in this regard nowadays. Many religious groups are trying harder to address this need.
4) Many religious groups have particular expressions of faith using media such as music and the other creative arts. This element alone can be comforting, enjoyable, and inspiring. (an example for me would be listening to the music of Bach, or to simple a cappella choral songs in an acoustically-perfect church building).
5) Religious buildings can be soothing, comforting, calming, safe, beautiful, and meditative. A physical place which helps calm the mind.
6) Certain religious practices and symbolism can become calming, meditative habits that teach one to relax the mind, be gently and quietly, reverently present. It is a form of relaxation therapy, yet imbued with a stronger sense of meaning in most cases, and therefore can be more appealing and effective.
7) Religious involvements can help frame major life events, such as births, marriages, and deaths. The community can come together in celebration or in grief. These events then can become accepted with greater meaning, and less loneliness.
There is one main disadvantage to "organized religion", in my opinion:
Dogma. When an inspired piece of wisdom or a metaphorical truth is understood as a literal fact, it becomes dogmatic. It would be like reading a fascinating, insightful, and enjoyable novel, but then starting to believe that the events in the novel are literally true, and acting accordingly. Many religious groups are quite dogmatic. The problem here is that dogmatism is an innate psychological tendency, which leads to different groups opposing and fighting with one another. It is understandable that most religions become dogmatic, because the founding of the religion and its texts is usually based on characters who really lived and stories which really happened -- it's just that the characters become idealized and the stories become more legendary and fictionalized over time.
The focus on dogma tends to distract attention away from whatever metaphorical truths may underlie the dogma. It would be like reading a fairy tale in a concrete or literal way, without considering whether there is a "moral to the story". Religious ideas can then also become judgmental and paternalistic, phenomena which can add to the already robust burden of self-judgment and self-criticism experienced by those going through a mental illness.
One can see in the world today a lot of religious dogma, leading to a lot of fighting about religion, all the while some of the core wisdom, such as "love one another", etc. falls by the wayside. Through history, a substantial portion of large-scale and small-scale human cruelty, catastrophe, political manipulation, and war, have been driven at least in part by religious dogmatism (even if seemingly well-meaning). We don't have to look far in today's news to find ongoing examples.
Dogmatism, from a psychiatric perspective, is fed by a variety of innate human personality traits, such as "obsessive-compulsiveness" (the tendency to require very clear, strict, or rigid pronouncements about what is right and what is wrong); also many dogmas are fed by narcissism (those who proclaim dogmatic statements are often doing so arrogantly, egotistically, forcefully, unempathically, in a grandiose way, or with an intent to control). Even without these two traits at play, it can be psychologically comforting to pronounce something as an absolute truth, because it may soothe the uncertainty and fear we may have about a variety of deep issues (such as dealing with death or mortality, finding meaning in life, explaining senseless tragedy, etc.). The difficulty is that the soothing effect may occur even if the "absolute truth" is an arbitrary--and fictional-- dogmatic pronouncement.
Ironically, some of the poignant themes in major religions such as Christianity, or Buddhism (others too, I suspect--though I do not feel well-enough informed to list them), encourage humility, gentleness, openness, acceptance, and encourage us to move away from obsessive-compulsiveness, narcissism, and absolutism in our thinking. Unfortunately, many self-proclaimed adherents of these belief systems may not actually embrace, perceive, or live out these themes. I suppose, within any set of beliefs, individuals may "pick out" selective elements which happen to suit them, while perhaps missing a broader perception of the whole. (I recognize I'm being a bit judgmental here, and I need to continue examining my thinking on this matter, to prevent my own dogmas from entrenching themselves)
So I think religion can be quite positive, with certain provisos. Nowadays, I do find that there are opportunities to participate in something religious without having to be dogmatic.
Here is a link to a recent Canadian Journal of Psychiatry article on religion, spirituality, and mental health:
While these articles are quite enthusiastic about the role of religion in mental health, I should point out several confounding variables:
1) Those who are more religious may also have more conservative beliefs, and a more conservative lifestyle. These traits are likely to be partially heritable, partially learned or chosen. This conservatism may protect individuals from various forms of life adversity. The problem is, many individuals do not fit into a conservative lifestyle paradigm, and may feel strongly excluded. Furthermore, the health of society as a whole would be strongly compromised by having such uniformity or constraint in lifestyle variables. We can look to nations with very strict moral or religious codes to observe the decrement in cultural and intellectual life that results.
2) Other lifestyle factors among the more religious may include a stronger focus on community, stable relationships, healthy diet, less substance abuse, etc. -- all these factors could mediate better mental health, rather than the religious faith per se. (From my own personal point of view about "God", though, I consider factors such as community, relationship, care for self & others, healthy lifestyle, etc. to be equivalent to "relationship with [or love for] God")
3) Those who already have better mental health may be more likely to form a stable, long-term relationship with religious (or other community or group) involvement. Thus, the relationship between mental health and religiosity may be associative, not causative.
Stepping out of this critique, though, I do genuinely believe that religious involvement is likely to benefit mental health directly in many cases, for the other reasons I've summarized above (e.g regarding community, meditation, friendship, support, having a setting to contemplate moral issues, etc.).
For some people, religion will not be "their thing", and in that case, I do think it will be important for them to find other sources of community, altruism, meditative calm, etc. Hopefully there will be more cultural development in this area in the coming generations.
As a recent addendum (today in April 2015), I have become a great fan of Richard Dawkins as a scientist and writer. I had been hesitant to read or discuss some of his work which specifically addresses religion (such as The God Delusion) but having read this recently, I have to say that I don't find his work very controversial at all. He summarizes a lot of reasoned discourse and insightful historical summary of quite convoluted, biased thinking that has influenced religious belief and practice for millenia. Richard Dawkins' greatest gift, though, in my opinion, is that he is a wonderful storyteller. In some ways I think he shares this talent in common with some thinkers about religion or mythology, such as C.S. Lewis or Joseph Campbell: in Dawkins' case, his best stories are about the joy and wonder of the way life works, in terms of genetics, biology, and natural selection. Dawkins is very passionate about science, and has become very passionate about challenging dogmatic belief systems which obscure the pursuit and joy of scientific understanding. In fact, he as well as others such as Stephen Pinker, show that obscuration of knowledge through dogmatic or mystical belief systems is a major hindrance to the health and peacefulness of society, and a major unnecessary cause of strife and conflict in the world. One element about religion, though, which Dawkins may not have attended to enough, is of the tendency for the brain to project idealizations or personifications of issues and desires, as a core element of religiosity, which then could be experienced in a psychologically healthy way, particularly if combined with a supportive community, tradition, and adornment from the creative arts. It is a human psychological capacity to personify metaphors or ideas, and treat them as external characters. I think it is easier to adapt existing religious cultures, to maintain positive elements of these traditions and possibly beneficial meditative practices and opportunities for ethical reflection in religious services, while moving away from a focus on dogmatic or fictional mystical beliefs. In this way, religious practice could move away any sort of conflict with science. Otherwise, there is very little at this point in atheistic culture which offers as much focused, organized opportunity for supportive community, meditative reflection, altruistic involvement, ethical discussions, infused by great art, music, and architecture.