Atheism, Theism and Agnosticism

Being in conversation with anyone sooner or later the topic of spirituality pops up. Usually I think it is enough to state ones believes and maybe discuss interesting aspects or maybe just do the later. Some people however seem to believe to have conquered truth. In my personal environment these hard believers are surprisingly mostly atheists. Anyhow I have read and heard about theists of any kind, be it christians, muslims or jews, trying to crusade the world. Sometimes I am strongly wondering why these people take their believes so serious.