I can't really name one major world religion with millions of adherents that tends to smile upon the use of psychoactive drugs, no matter the context. With the exception of Judaism, which uses alcohol as a ceremonial entheogen, and the Rastafari, who use cannabis similarly, the whole idea of incorporating a drug into a devout religious life is rather unpopular.
Here are some reasons I've come up with as to why. Which, if any of these, is the best explanation?:
1) Religions tend to be all about cultivating discipline. The presence of drugs makes this more difficult for MOST people, due to mental "clouding".
2) Drugs compete with religion. That is to say, many people who take drugs and have certain experiences and epiphanies that they would interpret as mystical or at least otherworldly. Thus drug users are more likely to see religion as unnecessary and not take part in it. Thus a ban on drugs is a survival adaptation for religions, in order to retain members.
3) The realizations people can have on drugs can cause them to see through the lies and the agendas for power, control, and the submission of others that most religions possess. Thus, as above, it's in religions' own best interest to discourage drugs.
4) Drugs cause people to see the world as a more complex place than religions tend to make it out to be, and thus to ask difficult questions that religions aren't prepared to answer.
Stoned Homer Simpson: "Could God microwave a burrito so hot that even he couldn't hold it?"
Ned Flanders: "Wellsir, as melon scratchers go that's a honeydew!"
5) Unlike the mystical traditions out of which they arose, religions tend to thrive on instilling certainty in people. Drug-takers know that our sober state of mind is only one of many ways consciousness can go, and thus are more likely to see things like morality and value as relative.
6) Drug-induced behaviors blur the lines between a sin and a good deed, an action originating within and without of onesself, and ultimately, the soul and the brain.
7) Perish the thought that drug users may come to find the fantastical visions they have whilst intoxicated more meaningful and relevant to their lives than the myths their religion teaches them, i.e. someone else's fantastical visions from centuries ago.
Feel free to add.
