Eventually everything fades away and gives its way to new things. This is particular true for religions. In Europe all ancient religions turned to mythology and stories for kids. There were systems of worship, power & politics based on those and sometimes, just like Christ, there were actual people in flesh, behind some of religions' birth & sacred saint or prophet-like people during religions' lifetimes. Jesus, for one, wasn't the first to be killed or even resurrected, the concept is quite old.
Take Delphi site: it has been the cradle for 3 distinguished religions, with centuries separating each other: Gaia, Dionysus, Apollo. With that order. Gaia signifies matriarch peoples & quite ancient, while Apollonian religion signifies a rather a patriarch society and brought by Dorians.
Religions, just like people, kingdoms, countries even like mountain ranges are born, mature and go. Nothing is to stay forever and all does.