For the wrath of God is revealed from heaven upon all ungodliness and unrighteousness of men, those suppressing the truth in unrighteousness (Romans 1:18).
The wrath of God here is directed at Christians; “suppress” is the Greek katecho and means to “hold back”, or “restrain”; now it is clear that you cannot restrain something unless you have it and you cannot suppress the truth unless you know it. This warning, then, is directed towards ungodly and unrighteous believers, not the godless people of the world.
How do we suppress the truth in unrighteousness? Each of us must ask the Lord to show us. But typically, for example, it is by being at home in this world; by seeking to be comfortable in this world, instead of choosing, acknowledging and pledging to be strangers and pilgrims (Hebrews 11:13); by being friends with the world, where friendship with the world is enmity towards God, and ignoring the fact that those who seek to be a friend of the world become an enemy of God (James 4:4).
We suppress the truth in unrighteousness by having much knowledge and little virtue; by confessing the truth with our mouth instead of with our lives; by not taking Christ at His word and trusting Him for everything. This amounts to not loving the truth so as to be saved (2 Thessalonians 2:10).
The human body is designed in such a way that food and drink is taken in, the good used to sustain life and the useless waste being automatically excreted; in the same way, the human soul is exposed to the continuing corruption of worldliness and ungodliness that is meant to draw us away into death. The difference is that there is no automatic function of the soul to excrete the evil; we must continually be doing it ourselves by an act of moral determination. As Paul wrote to Titus;
“For the grace of God was manifested, bringing salvation to all men, teaching us that we should live in the present age sobermindedly (i.e. showing self control) and righteously and in a godly manner, renouncing ungodliness and worldly desires, looking for the blessed hope and appearing of the glory of our great God and Saviour Jesus Christ, who gave Himself on behalf of us, so that He might redeem us from every kind of lawlessness and purify to Himself a special people, zealous of good works”(Titus 2:11-14).