Godlessness, Unrighteousness and Suppressing the Truth
The "truth" in verse 18 refers to anything that can be known about God. His wrath is directed against suppressing what can be known about God. It is like pressing down on something that will not stay down. It can only ever be an attempt to suppress the truth; it will never be successful. We have more than general revelation; we have God's word. How much less of an excuse do those have who hear His word week after week?