First and foremost, the philosophies driving this story's themes are very powerful and profound. This book had me digging deep on my own outlook on things like self-purpose, the meaning of love, and the institution of religion. I don't think the author had an agenda one way or the other about faith, in particular, but the dialogue is definitely meant to provoke and question.
The plot is divided into three interweaving stories, but the center of the storm is Hasan ibn Sabbath, the purported 'Old Man of the Mountain'. Hasan desires world domination. In order to achieve this end, he turned to a certain human condition known as religious fervor. Since religion and faith is something that one doesn't question even in today's context. (Try to convince a Muslim there isn't an Allah or a Christian there is no God, see what happens!) His methods go unquestioned.
In brief, Hasan takes in a young, but naive, strong teenagers and indoctrinated them with his own brand of dogma, Ismaili doctrine, an off shoot of an Islamic sect that believes the Prophet Mohammad's son-in-law, Ali, is the rightful heir to the religion. After much training that is more akin to Special Forces training in the present context, these elite force were dubbed the 'fedayees'.
After a valor display of battle glory, these fedayees were given a taste of paradise. Through subterfuge and vile machinations, Hasan managed to create a sense of martyrdom's reward in the impression of luscious gardens filled with 72 'hours'. To accentuate the experience of ecstasy, the boys were given 'hashish' or concentrated opium rolled into a pellet to swallow.
With a combination of drug-induced hallucinations, alcoholic intoxication and the physical touch of a female, these boys are being 'poisoned' psychologically and the craving for such experience is so over-powering that they would do anything to go to 'paradise' once more.
The reasons I love this book is because it shows with perfect realism how religious fanatics were developed and maintained , how harems were built and worked, and how higher-ups used knowledge--or rather, the lack of knowledge about life and spirituality and human existence in general, to steer the minds of young, naive men and sustain dogma, thirsty for blood and revenge, but it also kept me glued with its story telling. A lot of times it actually saddened me. The horrors and inhuman an individual can do/be. I understood the main character but cannot decide who really is worst off. No believe should demand the death of others.
Finally, one should not it read as a historical document. It is, was, and will always be meant to be a work of fiction, and a pretty good one.