Historically speaking, humans have had a real knack for identifying superficial differences in people and separating them into categories based on those differences. This system is a very effective means of discrimination, because once it is clear that two groups are different, it becomes easier to make arguments (ridiculous though they may be) about which group is superior. Differences in religious belief have caused some of the more dramatic incidents of division and discrimination throughout the course of civilization—I’m looking at you, Crusades—but separating religions themselves into categories can have more subtle and long-term effects on culture.
With each new generation of believers, there is a slow evolution of “old” and “new” beliefs. Once-thriving religions, especially Pagan religions, are now either shunned into the realm of mythology or considered to be hokey counter-culture territory. This is a distinction we see mimicked in fantasy worlds. Even in alternate universes or histories where magic is plainly observable and actual deities occasionally turn up in unquestionable physical form, there is often a distinction between the “old” and “new” religion, and with that distinction comes a division of people: those who follow the old gods, and those who follow the new. This distinction typically comes with some indication of which religion is supposedly superior: in some narratives the old gods are benevolent and powerful, and the new gods are forcing them out of their rightful dominion, and in others the old gods are wicked and archaic, and the new religion eclipsing them changes the world for the better.
A good example of this is in George R.R. Martin’s series A Song of Ice and Fire and its accompanying television show Game of Thrones. The religion currently most in vogue in the fictional world of Westeros is the Faith of the Seven. This prevalent new religion—though it is already technically thousands of years old at the time of the story—stands in contrast to the vague but eponymous “old gods of the forest”, who are worshipped largely by those in the far north, and with waning enthusiasm by “civilized” people. When swearing in new recruits to the Night’s Watch, Jeor Mormont asks “Does anyone still keep the old gods?” to which only Jon Snow replies in the affirmative.
Though there is little outright religious persecution in Game of Thrones, it is widely understood that the old gods represent an archaic religion, and because so many of the worshippers of said gods are uncivilized “wildlings”, the religion is treated with a sort of grudging respect in more civilized parts of the world.
This is most similar to how Pagan religions are treated in the modern world. Many people take an academic interest in Greek or Norse mythology, for example, but actual practitioners of these or similar religions are seldom taken seriously by larger faith communities. This is partly the result of the evangelical nature of Christianity, whose followers worked very hard to destroy Pagan religions by force or by conversion. This process was made easier by deliberately associating Paganism with barbarism and violence, creating negative associations with the “old gods.”
Because this relationship between the old and new gods parallels the relationship between “modern” and Pagan religions in reality, it is quite common in fantasy. Neil Gaiman’s American Gods follows—in a very literal way—old-world gods coping with their diminishing power in the face of new gods (in this case things like television and internet). The Lord of the Rings handles religion somewhat less explicitly, but the elves and their mythic ancestors are deity-like and connected with the earth and nature. There is strong emphasis throughout the series on the fact that the reign of elves is coming to an end and the time of the short-lived, ambitious, and mercurial human race is beginning. This is not necessarily a terrible or tragic thing. However, where Game of Thrones tends to portray the old gods as dark and frightening, The Lord of the Rings takes a more melancholy tone at the idea of magic leaving the world forever.
The dichotomy between the old gods and the new in fiction is likely so pervasive because people in the modern world both see this clear distinction between living and “dead” religions and feel ambivalent about it. The typical church-going Christian might scoff at someone who worships Jupiter, but if Christian forums and websites are any indication, it’s very common for God-fearing people to take interest in the zodiac, in spite of its connection with Paganism. Christians sometimes keep good luck charms such as rabbit’s feet, another Pagan concept, even though Christian dogma disapproves of such talismans.
The fact is that people feel a sense of power and mystery associated with things they consider ancient or otherworldly, and thriving religions—no matter how long ago they were technically founded—lose this sense of mystery when they are embraced fully by the modern world. Even when the new gods promise happiness, miracles, and a peaceful afterlife in exchange for peoples’ devotion, the old, dead gods still lurk in the shadows, armed with a power the new gods somehow lack, and that power remains both frightening and alluring.