How to discuss religion with believers?

[Dr H]

Paul is addressing ungodliness.  Atheism is nongodliness.nongodliness' - Haven't ever come across that before, but moving right along... 

Nongodliness - Haven't seen this before, perhaps made-up, but moving right along.

Quite simple, really.  "ungodly:" literally means disobeying God, or going against God.  
An atheist does not believe that there are any gods.  
It is impossible to disobey or go against something which one doesn't even believe exists.

"Non-" expresses negation or absence.  Hence "nongodliness" = "acting in the absence of god".

19 For what may be known about God is plain to them, because God has made it plain to them. 

Saying this does not make it so.

And then, creation itself is the evidence: vs 20 - For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse.

IOW, "God exists, because creation exists, and God made creation."

This is circular reasoning, and therefore logically invalid.

Without scripture, it would be concluded atheists actually believe what they are saying about God's existence.

I find it difficult to parse that sentence into meaningful concepts.

What an atheist says, is that they lack belief in any deity.
That would be a simple statement of fact about what they do not believe.
The existence of scripture is completely immaterial to that state of affairs.