In traditional Christianity, and in Tolkien's writings, the decline is not just a matter of myths and legends, but it extends to history. There used to be a great ancient empire, but it fell apart. You can still see its remains everywhere you go, because nothing that came afterwards managed to reach its greatness.

The success of European colonial empires started challenging that worldview, but some of the ideas remain strong. Especially in the parts of the world where the dominant church still claims it's the one universal Christian church. Of course, if you are a descendant of Protestants who fled Europe, your ancestors deliberately left that world and that culture behind.