I have been told that as an atheist, I'm as closed minded as those who believe a god exists. That the only open minded stance is to be agnostic.
I disagree with this.
I am an atheist in the same way that I'm an a-unicorist, a-lochnessmonsterist, a-bigfootist.
These are all things that people have imagined at some point in human history because of something they saw or experienced, and tried to make an explanation for them. From every single position of proof, unicorns, the lochness monster, and big foot don't exist. So, I don't believe in them. I don't think they exist.
Yet, I'm open to proof. Show me that these things exist, give me proof, and I'll change my mind.
Being certain that something doesn't exist because there is no evidence for it is not closed minded. Because this understanding is based on evidence, or lack thereof, there is always room for a different opinion if the facts change.
Being certain that something exists without reproducible proof strongly leans towards the possibility of closed-mindedness. Because there is nothing anyone can do to disprove further that this thing doesn't exist. There is no evidence to begin with, so there is no way to decrease the evidence any further. There is no room to change one's mind because the perspective is not evidence based. There's no way to take away the thing that got the person to believe in the first place.
That is the development of a closed mind - basing a conclusion on something that can't be challenged. Basing a conclusion on a feeling, on something that is not reproducible, or on a story makes that belief iron-clad. And therefore, closed.
Basing an opinion on the facts that we can see - there is nothing there, so until I see something, I do not think anything is there, is not a closed minded position. It's not a position based on a story or a feeling. It's a conclusion derived from evidence.
Needing a lot of evidence to be convinced of something is not being closed minded, either. If someone tells me one thing that is proof that a unicorn exists - say, "I saw one once" or "There's a picture on the internet" it's not enough evidence to prove they exist. Even seeing one in person, after never having seen one before despite them being in our cultural story for decades, would not be enough proof - is that a real horn?
Skepticism is not a closed mind. It's a mind that doesn't change easily, but it's not closed. A closed mind is one that has a story without evidence, and goes with it and will not consider outside evidence when presented.
As an atheist, I consider outside evidence constantly. I consider it and each time come to the conclusion that given the evidence, there is no God as humanity portrays him/her/it.
There has been one argument for a "God" that I can accept, and only one so far - God is actually the entire universe. It's not a "being" so much as the dynamic of an entire system working together - it has no consciousness like we know, and it doesn't have an intention, but is instead the name they give the universe. That, I can understand. Because we can see the universe is a gigantic living eco-system that is far more complex than we can understand right now, and more complex than we may ever understand. Using the term "God" to describe just how big and impossibly complicated the universe is, I can get that.
All other depictions of God, it being a sentient being of some kind, or there being some kind of human-like afterlife, I don't believe. And I won't believe until we have evidence that it exists.
That, to me, is not closed minded, but prudent and drawing conclusions based on evidence. I am willing to change my mind. Prove to me that unicorns exists, using real evidence, and I'll change my mind.