The Christianity we know today was the state religion of the Roman Empire. It's core doctrines were agreed on in a council summoned by Constantine the Great. But then the great ancient empire fell. It was replaced by many lesser kingdoms, which tried to keep some old traditions alive, and the Church was the unifying force. But then the Church itself split apart, and the last remnants of the old empire fell to infidels coming from the east. And then the Church split again.
The idea that the world becomes lesser over time used to be a core part of Christian worldview. Industrialization changed that, but Tolkien considered the consequences of industry and technology a great evil.