Read Romans 3:9-18
Romans 3:18 is not speaking of a reverential fear of God that a person has who recognizes Him as the great Potentate of all ages and as the Almighty God we serve.
Rather, this verse refers to those who have no concern for the existence, character or attributes of God. They do not think that God merits any thought at all. They completely fail to recognize their accountability to Him.
People's basic problem--the root cause of all their trouble--is that they do not know God, and they do not fear meeting God when they die. People speak lightly of death because they do not want to face its realities.
People have taken it for granted that God, if He even exists, will overlook what they do and will take care of them, regardless of how they live.
People's refusal to make God the God of their lives is the fountain from which all these evils flow. Solomon said, "The fear of the Lord is the beginning of wisdom" (Prov. 9:10).
When people refuse to fear God, or recognize Him for who He is, they lack wisdom, and they experience increasing mental confusion. One needs only to consider the fields of modern music and modern art to see this.
And in addition to the absence of wisdom and an increasing mental confusion, there is also moral and spiritual darkness.
"The fear of the Lord is the beginning of wisdom: and the knowledge of the holy is understanding" (Prov. 9:10).