"The fool has said in his heart, 'There is no God'" (Psalm 14:1).
Naturalism or materialism is the view that nature is matter and matter is all there is. This is an ancient view that may be said to have begun in the West with Leucippus and Democritus (5th century BC), and in the East with Buddha around the same time. Mechanistic causes account for all changes; there is no mind, and no spirit. Atoms alone exist. The intellectual effort to reduce all complex processes to simple terms such as matter in motion is called reductionism.
A version of this view has become the prevalent philosophy of our times, partly because of the success and consequent prestige of physical science, which studies the physical world. The exaltation of science as the only ultimate authority is sometimes called scientism.
It is not possible to present here all the arguments against reductionism and scientism. One can simply suggest that science is insufficient to account for the scientist: in other words, for human aspirations for meaning, purpose, significance, identity, peace of mind, love, and hope. All these categories must remain undefined or reduced to chemical or mechanical processes for the naturalist. God alone fulfils these aspirations. This is why David refers to such a person as a "fool" (Psalm 14:1).