Greg talks about Romans 1:18-19.
This question is simple: Are atheists just suppressing the truth in unrighteousness? This is a phrase that comes from Romans 1. My first response was, “no duh.” Yes. Read Romans 1:16 and following, and it’s straight-ahead clear. What Paul says is that the evidence of God is clear to everyone having been seen through what has been made.
You look around the world, – natural revelation, general revelation – and you realize something is going on that's greater than us. The thing that's going on is God's activity, evident to everyone. But the writer says in Romans 1 that men characteristically suppress the truth in unrighteousness. That is, they hold down what is trying to erupt up in an obvious way, like trying to hold a beach ball underwater. You have to work at keeping it down.
The evidence of God is so obvious in the world – that the world exists and the way the world exists (the cosmological argument and the teleological argument) – that a person has to be denying the obvious and aggressively pushing down the evidence to turn his head the other way and deny the existence of God.
Romans 1 is speaking certainly and most explicitly regarding atheists, but not just atheists. Some people acknowledge God but deny the God that's there. They exchanged the truth of God for something else. Instead of worshipping Him, they worship creatures, crawling things, and other human beings instead. All false religion and no religion are examples of repressing the truth in unrighteousness, and it strikes me that atheists would be first in line as an example of that kind of thing.