belief in the existence of One True God
the worst of the Model Theism-s - a Religious Cult
mainly Judaism, Christianity, Islam
Big Religion-s may have had some value as counterweights to big nasty tyrants: it's always good to avoid a Monopoly. But since the First World has pretty much shaken that off, it's really indefensible.
twin to Fundamentalism? Both seem to require a resistance to evidence. Perhaps Fundamentalism is simply the honest unapologetic face of Monotheism.