It’s almost always a simplification to point to a single ideology as being “what the culture believes.” With that caveat, however, it is not inaccurate to say that the opinion-makers of America–academia, media, scientists, etc.–have nearly unanimously embraced the naturalistic worldview. While even its supporters struggle to define naturalism precisely, at its heart is the simple idea that everything in the world (both what exists and what happens) can be explained through purely mechanistic cause and effect. Everything from planets to animals to ideas is ultimately the product of a chain of exclusively material “dominoes” stretching back into the unknowable past. The theory really came into its own in the 19th Century, as Darwinian evolution purported to fit the diversity and apparent design of biological life into that same impersonal progression of cause and effect.
Naturalism matters to a Christian because it is what is left over when theism is discarded. Throughout history, humans have assumed there are two fundamental sorts of “stuff” in the universe: mental/spiritual existence (things like gods, angels, ideas, and values) and material/physical existence (things like elements, molecules, and atoms). Christianity and most other theistic worldviews assume that the spiritual existed before and was the cause of the material: “In the beginning was the Word… All things were made through him, and without him was not any thing made that was made.” Atheism, on the other hand, by definition cannot accept a preexistent and creative Mind. This leaves the atheist with a world in which everything begins with impersonal, material being; naturalism, in other words.