Why was there a lot of gods before and now the are not?

For example the aztecs believed in Quetzalcoatl the Feathered Snake and other gods and when The spanish conquered Mexico and spread Christianity, and all of those gods disappeared, but werent they real to some people, and what if one day another religion conquers all the places with cristianity and puts their religion in, god is gonna stop to exist and that new god of that new religion s gonna be the new god and jesus and christ are just gonna be another god that supposedly existed, remember back in the day every one thought Zeus and all of those greek gods were real, and look what happen to them, so what maked god any different???