Sometimes people seem to forget that theists are overwhelmingly quite normal, intelligent, and caring people. They are normally just carefully indoctrinated into believing in some untrue and sometimes admittedly even quite silly ideas.
People like to do that because sweet lies do so often give sweet comfort. One need not to face the some truths like your own mortality anymore. The humanly invented idea of an afterlife can take much of fear of death away.
However, this happens only as long as a person can retain a belief in this fragile idea and many people see a direct need to protect it. One can also believe in a "god" that guides and protects a person when the real dad is incapable of protecting and guiding one anymore.
It is easy to understand why sweet lies like this can gain so immense popularity even among other ways quite sane people. Most religious ideas are carefully crafted to be as immune to rational inspection as possible. People mostly just love to be told sweet little lies in all fields of life.
Most of all people are taught religious ideas in an age when they still trust everything an adult tells them. After these ideas are repeated time after time, their utter familiarity makes them believable in a new way. This is the case, even if the often quite similar beliefs of a neighboring religion can sound even laughable. Many people also have been duped into believing that morality and ethics can come only from their pet religion.
Many conservatives love the strong social conservatism that is part of almost all religions. Throughout the history, most religions have associated themselves with the needs of owning and ruling classes, and they have always served them well. However, there is nothing insane or stupid in this. People tend to love ideologies that do serve their interests.
There is a growing group of people who really are not even theists. They do embrace a religion as a part of the package of some (normally national) cultural heritage. They often believe that even an ancient religion can still be important even in a society that is extremely different from the one in which it was born.
A major problem is that these "cultural religionists" often end up accepting ideas that confirm their decision to support a religion. In practice, their actions and ideas can be quite like those of "real theists". Most of all they all too often see the opponents of their religion as also their opponents. This can be the case, even if they do not really believe in many of the odd supernatural parts of that religion.
Naturally, one cannot forget the immense forces for social cohesion that do protect all ruling religions, and even many strong minority religions and cults. When a religion gains an upper hand in society or in a local community, the social pressure not to ever even doubt it in any way can become immense.
This social pressure is often even the most important reason for the continuation and a strong position of a religion. Without this pressure, humans would follow a very natural tendency to forego strong ideologies. Most people would live their lives as humans have always lived; in a way that would cause them the least trouble.
All successful religions do contain large collection of myths, ideas and practices that can take hard work to learn. After investing much time and effort in learning these thing, many people end up defending their major mental investment in a religion.
A paradoxical fact is that the more difficult a religion is to master wholly, the greater the mental investment can become. Quite sane and intelligent people can end up defending a earlier heavy investment even against their own rational thinking.
Sometimes quite harmless collections of small untruths and lies can grow to become gruesome systems of self-deception. They can become even engines for repression, cruelty, and hatred. This normally happens when a religion grows to the status of a state-sponsored ideology.
However, even the people who do sell and market these ideologies as their paid full-time job are not normally insane or stupid, even if some extreme cases they can seem to be so. Of course, even the most insane sounding extremists can fake their extremism if they are smart enough.
When people choose a religion as their career and source of livelihood, some of them just will often want to be beat the other professionals even in that field of life. The human tendency for wanting to be good at everything we do plays into the hands of religions too.
People have always invented new ways to be better believers that their co-believers. When intelligent people do this, they can also come up with some quite clever new ways for forwarding their pet religion. The existence of a paid class of clergy is normally a major reason why religions do live on as strongly as they do. A person will always fight and work to retain his job and future source of livelihood. This is more so if this profession is the only one for which he has education and qualifications.
The clerical profession is often also a quite pleasant one, when one has learned to endure sorrow and grief that is associated with some religious rituals. Many sane and intelligent people will keep up the appearances, even if they have no real faith left. However, even a fake professional believer will help to keep the religion going.
The teaching of the future clergy also normally employs a lot of professional people, who also have a strong vested interest in forwarding the religion. These people are normally quite sane and intelligent. The need to protect the source of their livelihood does often guide even their thinking. This is the case also in very many other professions too. This process is often subconscious.
Religions are commonly used to define and even create strong and durable communities. Strong dietary regulations are often used to good effect in this respect. A person who has, for example, learned all his life that eating pork is somehow disgusting will easily see those who do eat pork as disgusting as people too.
A common belief in things that are hard to believe is a also strong bond-creator. Paradoxically enough the more hard to believe a believed thing is the stronger is the bond that the belief in it can create. This is one of the reasons why a fully rational religion will quite inevitably lose to an irrational one. A rational religion is unable to create similar community and bonding as a more irrational religion can create.
There are many other methods that the religions can use to bolster their position in a society. Blasphemy laws often prohibit all serious discussions over these human ideologies. Religions are then allowed to hide behind a self-generated claim of "divine origin" that is supposed to make their critique impossible.
This is a common policy because it is a fantastically effective one. It can bolster the position of a religion in a society even immensely. Also in societies where there are no blasphemy laws any more, the keenness of the true believers can make real public discussions over a religion so hard and unpleasant experience that people will soon learn to avoid them, which is often the goal in the first place.