I'm not an "atheist" per say, but someone who doesn't believe in the existence of an all powerful creator god in any way.
I find the question "how do you justify" to be a bit offensive actually. No one needs to "justify" their beliefs nor lack of beliefs. That's the beauty of the freedom of religion.
We are free to believe what we believe, no justification really needed.
So the better question is simply "why?"
"Why are you atheist?"
Since the question often really means "Why don't you believe in my god?" or "Why do you believe my god does not exist?" I'm interested in replying.
And then I bump into this problem.
"unjustifiable"
Well, there's a bit of a problem with this. Similar to the one before. I don't think you're using the "right" word.
What would probably be a better word is "unprovable".
Right, with those complaints out of the way.
I find the idea of a single creator god, responsible for all existence, to be a bit of a cop out.
See, when I was a child, I was raised in "Christianity light", you know what I mean. The bible existed in the house but wasn't read. God was presented as a benevolent protector and creator of mankind. Sunday-School never mentioned hell, and taught the many tales that were supposed to show how good and generous god was.
And I, as a child, found myself with questions.
I think the first thing that made me go "wait, what?" was a short song.
I don't know if it exists in English, but the lyric in Icelandic tells the story of two men. One was stupid and built his house on sand, the other was smart and built his house on a mountain. A flood came and wiped out the house on the sand but spared the house on the mountain.
They told us that the song was about how faith in god would keep us safe. That the mountain was faith.
And I remember semi-obsessing over this song.
I realized I didn't have "faith". I didn't walk around in this certainty that something other, greater than me, existed. I didn't even trust that something other, greater than me, existed.
I had been told that this powerful thing had personally created me and all things, and that he personally was watching over me and all things, and I had been told that I needed to have faith in this as truth to be safe from a metaphorical flood.
I found myself wondering. If yhvh made the stupid man stupid, and loved the stupid man as he loved all men, then why would he blame the man for being "stupid" and wipe his house off the sand?
This was my first serious question, and no man has ever provided any good answer to it. The only answer I've gotten is the equivalent to "God works in mysterious ways"...
See, here's why the "all powerful creator god" is a copout to me.
Everything that happens "god did it this way".
We find evidence that it happened in another way, then "god did it anyway".
God is good and loving and benevolent but bad things happen, then it's all a part of "god's plan". God's plan doesn't make sense, then "you can't understand god", "god works in mysterious ways", etc.
There's no rhyme nor reason provided.
Belief in an all powerful god seems to depend on the idea that you can't understand an all powerful god, so everything you know doesn't matter, cause god's "greater" than that.
And you know what,... I don't buy it.
It just doesn't have any logic behind it.
If there is an all powerful god who wants to be believed in, then why would he create doubt and disbelief?
If he doesn't want us to believe in him really, then why does it matter that I don't?
How can a person believe in an all powerful, all knowing god, and at the same time, believe in free will?
Anyway.
While my existence is an Operating Assumption* to me and little else, I still prefer, under that assumption, to refer to logic and reason.
I like evidence.
I like peer reviewed research.
I like things I can rely on.
I base my beliefs and opinions on as much logic and reason and Evidence as I can.
Evidence being the key word.
There's zero evidence that there's an all powerful, all knowing, omnipresent deity, so, I don't have any reason to believe one exists. Thus, I don't have belief in his existence.
I am not "agnostic" per say. I don't stand here going "I don't know whether there is a god", even though I don't know. I'm not on any proverbial fence.
I sit here and go "You know what, there's no evidence that this thing exists, talk to me when there is".
And yet, I'm not an atheist.
Why? Because I make the conscious choice not to be.
I make the conscious choice to work under the operating assumption that the gods I personally prefer have some sway over my life. I don't know if they exist, and I have no "faith". I have an operating assumption.
(*Operating Assumption; Operating as if the Assumption that X is correct.)