"For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness" (Romans 1:18).  This verse highlights four fundamental facts.
First, there is such a thing as truth.  Scientists search for it in their sphere.  Doctors heal by it.  Judges render right verdicts by it.  Why should the knowledge of God be any different.  Agnosticism is a cop out.  Truth is never relative - certainly not here.  Central to the knowledge of the true God is the Gospel of Jesus Christ, Who came to die for our sins and purchase salvation for those who believe.  "Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me" (John 14:6).
Second, God has given witnesses to testify the truth to men.  This is especially so in the written Word of God.  "Thy word is truth" (John 17:17b).  The most fundamental life-and-death questions of human origins, purpose, morals, destiny, and eternal salvation are answered by God's written Word.
Third, this world also harbours that which is evil, false, sinister, and ruinous.  Romans 1:18 speaks of those who "hold the truth in unrighteousness".  The word "hold" has the idea of hold down, or suppress.  We hear of those who are in denial about health problems, eating disorders, or marriage problems.  Yet here is the ultimate expression of the perversity of sin in the human heart - to suppress the truth that God has testified for our own deliverance.  How much better it is to be open and have brokenness of heart  before God and His truth.
Fourth, God's wrath awaits those given to the insanity of resisting the truth out of love of the false.  Our text warns of the possibility of being so enslaved to ungodliness and unrighteousness that one would even suppress the truth to their own self destruction.  God is willing to extend grace to those who repent through His Son, Jesus Christ;  But He has before revealed His sober determination to bring judgment upon all who persist in sinning against His Truth.