Lost what belief?
If they were saved, then read the Bible and chose to be an atheist, then I have to believe it's not because they don't believe in God ... it's because they're mortified at what they read.
It's a bit more complicated than you would feel comfortable with. Atheism to theists is very hard to understand or accept. It means a flaw somewhere in the system.
I sense that the way believers view atheists is more related to confusion and fear.
As Subduction points out it is the
null hypothesis. Just as the existence of the Biblical Flood should be studied.
Now I didn't start off an atheist but that's likely because I had no choice in the matter. I was, like everyone else in the midwest US raised from the cradle to believe there was some form of God, not a particularly fire and brimstone type, but some type.
In that way one doesn't really get a chance to start with the null hypothesis (Ho: "There is no God") and test against it. Most of us have to stumble our way around trying to figure out why the exercise is trying to find a God when He seems so very absent or logically impossible etc.
We start off with a faith given to us and grow up accepting it as real and we come to a point where we wind up trying to test if it is real.
I had to work quite hard to understand the faith I had been "given". I came to some conclusions chief among them that if there was a God there were definite RULES and if those rules were not adhered to to the best of my ability or (gasp) if I said, thought or did something wrong it might be very bad. Now mind you, I was never TAUGHT by my family or my church that there was a fire and brimstone God, but the word was out on the street. It could be picked up elsewhere.
As I grew older and the feeling that I wasn't really connecting with "God" in any meaningful way was taking ahold, I was left with the FEAR alone. Fear of this God whom I was failing to connect with.
The only thing I had was my ability to assess the situation. I couldn't change reality, but I could observe. And read. And try to understand. Try to understand what the Bible said, what the thinkers said, what the holy men said.
And in the end I simply felt that the FEAR was not offset by anything positive in my life. In fact I could explain as much of the variability in my world
without the God Concept and I could let go of some of the FEAR.
So here I am.
You can slice it and dice it and parse it as you wish, and the other Christians can pull it apart and tell me how I failed somehow to approach God in just the right way (like I was hunting a scared deer or something), but in the end if there is a God HE
holds all the cards, so if there's a disconnect I can at least feel like I did my part to the best of my limited abilities.
The Flood, on the other hand, is a great hypothesis to start from the null: "There was no Noachian Flood". Let's test
against that null hypothesis.
What do we see? ZERO evidence of that Flood, but a scrap of paper somewhere written by an unknown person claiming there was such a flood. Is that scrap a literal truth? Or is it a story? Is it a misperception by people with less knowledge of the rest of the earth than we have today? Hmmmm...all evidence points to it being a PERSON (or people) who wrote the story, so I'm going to assume that it
could be false and at best is insufficient to make the prima facie case for The Flood.
Like the God-concept, I fail to believe in the Flood as well.