If I were to throw out certain metaphysical presuppositions (I'm not a brain in a vat, I appear to be autonomous, my reality is what it appears to be, etc ), then every position following that would be unfounded and pointless.
You're calling the necessity of accepting certain things, that are required to form a useful thought or opinion about anything, a belief system?
None of those things have anything to do with theism and atheism. And the metaphysical presupposition that reality is what it appears to be is actually significantly more problematic for atheism than theism, at least for those who do not toss out materialistic theories of mind. Things get pretty wild over there, people start declaring that consciousness is an illusion, and then rationality goes out the window as reliable and every position
is unfounded and pointless.
The idea that God is a proposition that can and must be tested scientifically is
itself a metaphysical claim. Once we're dealing with metaphysics, we're in the realm of beliefs.
I don't think that all; actually the opposite.
If the logistics of how you got there prevent you from imagining and accepting that, yet, still you are there, that's ok.
You seem to have decided that if I approach atheism differently than you do, clearly I am still thinking from my current perspective, because otherwise I would agree with you.
I'm going to take a minor issue with this.
When I finally admitted I was an atheist the only thing that changed was that I didn't believe there was some agent behind the universe. That was it. Nothing else changed. For a while I even considered calling myself an atheistic Christian because there were many things about the church I liked. The only disagreement I had was that the supernatural claims had not been demonstrated to be credible. But I felt it was dishonest to call myself a Christian if I didn't believe in the divine inspiration of Jesus' message.
There were no other changes to how I viewed the world. As such I don't think that my change in position can realistically be called a change in world view.
Not believing that there's an agent behind the universe
is a metaphysical belief. I'm talking about that rather than ideals--it's pretty normal to maintain Christian values even after leaving the church. Whether they can be rationally defended is another question, but I think it's pretty common for people who leave Christianity to still view the world through something of a secular Christian framework. I wouldn't be able to, but many can. Which is good--nihilism is no fun.
What I mean about "belief system" is a bit different, though. If you examine why you came to the conclusion that there is no God, you'll likely find presuppositions about the role of science in inquiry, the possibility of miracles, and so forth and so on. You may not have had a change in worldview, but you likely had a very modern worldview to begin with. Which is not a criticism at all--there are reasons that atheism has flourished in the modern age and was really just an oddity beforehand, and it's because of the way we moderns look at the world. The problem is that our viewpoint is subjective and we've decided it's objective.
It is very hard to have a conversation when one side will admit to subjectivity and the other won't. This is why I dislike the atheistic claim to somehow being the default, because there's no such thing as a default position. There are different paradigms at work here in how we assess truth.