Religions don't stick around forever. Cultures shift, beliefs change, Roman emperors convert to Christianity...
Now you can experience this aspect of history in Civ4. With this mod component, whenever there are 5 or more religions present within the borders of a single civilization, one of them will be removed from the map entirely. Feel free to tweek the numbers as necessary. Obviously this component is best suited to a mod which contains a large number of religions, although no changes to the religions themselves are present in this download.
Enjoy.
Now you can experience this aspect of history in Civ4. With this mod component, whenever there are 5 or more religions present within the borders of a single civilization, one of them will be removed from the map entirely. Feel free to tweek the numbers as necessary. Obviously this component is best suited to a mod which contains a large number of religions, although no changes to the religions themselves are present in this download.
Enjoy.

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. From a gameplay perspective, this means you can plan around your religion's imminent demise and possibly delay it. It can also be used offensively; if you acquire a rival's religion you can research the obsolescing tech and eliminate it (although if the religion is no present in your empire you cannot obsolete it). From a flavour perspective this can be used to represent new knowledge out dating the superstitions of the past.
But it would represent the waxing of a religion quite well!