Years ago, I was in a hardware store and from a distance I saw someone I knew. Typically, when that takes place, I'll take the time to say "hello", but on this particular occasion, I didn't want to. The man I saw had just created a lot of conflict for some mutual friends, and I didn't want to engage him in conversation because I didn't want to get drawn into the drama. So I pretended like I hadn't seen him. I kept shopping and tried not to be seen. To my knowledge, he never knew I was there, but it felt very strange to try to walk through the aisles of the store while actively hiding from him and pretending I didn't know he was there.
In many ways, that's the way a large percentage of humanity lives their lives in regard to God. Instinctively, they know He's there, but they're doing their best to pretend He isn't while doing everything they can to avoid running into Him. There are consequences for doing this, and a dark side to living with this kind of disbelief that Romans 1:18-32 illustrates.