If one has a closed natural system where nothing can enter to support it from the outside, the system will run down. Any order will become disorder in a finite amount of time. Entropy is a measure of disorder.
In naturalistic or materialistic worldviews the universe we live in is supposed to be such a closed system. There is nothing outside it. Or, if there is, that supernatural stuff can’t break through the boundaries of the closed system to offer support.
In the Christian worldview not only is there an outside to the universe but that outside is powerful, willing and intelligent enough to both create the universe and make an ongoing difference to it. The material universe is not closed in this worldview although it could be hypothetically viewed as closed to study what would happen to it assuming the outside did not intervene.
For example, if we assumed nothing would stop erosion, weathering and traffic from the outside, we could ask how long the hypothetically closed system of the rock formations in the photo above would remain intact? No one doubts that there is intelligence outside this system of rock formations. Just by studying the system we are that intelligence outside it. After a study is made other people powerful and willing enough could use the findings to implement policies to preserve the formations. All of that study and preservation comes from outside this system of rock formations intervening to alter what would happen to it if those formations were left alone.
If we supposed that our universe were closed with no outside support, the main mystery would be how our universe came to be. With entropy we cannot claim that our universe had always been here to avoid addressing its origin. If our universe were infinitely old, it would have run down by now. Furthermore, any popping of ordered reality out of nothing or even out of less ordered stuff would require an explanation how that popping could increase orderliness without the assistance of some outside intelligence.
In the past it was believed that random mutations filtered through natural selection could serve as the mechanism allowing living organisms within a closed universe to evolve into ever more complex organisms without help from the outside. The change coming from random mutations would be mindlessly directed by natural selection to achieve this. R. A. Fisher even offered a mathematical proof that it would work.
However, with further study of mutation-selection the opposite is now known to happen. This perpetual motion mechanism is not what we thought it was. Even Fisher’s Theorem has been flipped by William F. Basener and John C. Sanford. The following podcast with John Sanford puts this in perspective.
Rather than describing a means by which evolution could occur mutation-selection describes the mechanism behind genetic entropy. Without outside help, living species in a closed system not only do not evolve into super-species, they run down just like everything else does. At some point in their genome degeneration they go extinct.