Manichaeism is an interesting case study. At one time it was one of the most popular religions in the world and then *poof* it was gone. Quoting Wikipedia:Manichaeism thrived between the third and seventh centuries, and at its height was one of the most widespread religions in the world. Manichaean churches and scriptures existed as far east as China and as far west as the Roman Empire. Like most religions, Manichaeism borrowed heavily from previous religions. Manichaeism borrowed from Judaism, Christianity, Zoroastrianism, and Buddhism. Mani died in prison while awaiting execution, very likely as a result of severe torture. Christ, Zoroaster, and Báb were also murdered -- founding religions is a dangerous business. Manichaeist's were considered Zindīq by the Abbasid caliphs, a title which was punishable by death. Eventually, the religion died out in every country where it had adherents. Adherents.com tracks over 4,200 existing religions, of which only nineteen have more than one million followers. Probably most of these will go the way of Manichaeism in the next few hundred years.