If you’re not a nihilist which philosophical foundation do you draw your atheism from?
Atheism derives directly from two beliefs, and is itself neither a belief nor a source for other beliefs. Atheism is the inevitable result of rational skepticism, or the belief that no claim should be accepted beyond what any available evidence supports, and that there iis insufficient evidence to accept any god claim or hold any god belief. It's that simple.
Consider avampirism and aleprechaunism, which are also neither beliefs nor the source of any belief, and which also are the result of only two beliefs each - a respect for rational skepticism, and the lack of sufficient supporting evidence for vampires and leprechauns.
You're probably an aleprechaunist, right? You are if you lack a belief in leprechauns. What is your aleprechaunism founded on? Nihilism? What beliefs derive from you non-belief in leprechauns?
being an avid atheist, which is literally soul-eroding:
You're projecting you own internal state onto the atheist. What you are telling others with a comment like that is how empty and insufferable life would be for you without your god belief, and that you assume that others must be the same as you would be without your faith.
You've been told repeatedly that that is not the case as common sense would attest. If the atheist had a deep yearning resulting from a lack of a god belief, he'd be searching for completeness and direction. Most of us are not. People that come to us with religion are routinely turned away.
Why do you suppose that is?
And if believers had found something special, it would be apparent and appealing.
Common sense will tell you that the atheist does not partake from that cup because he has no such thirst. This atheist couldn't be more content with his life. A lifetime of humanist values and methods has resulted in a life of love, friends, wonder, and gratitude.
What do you have to offer that can improve on that? The belief that the lack of a god belief being soul eroding? I've experienced life both inside and outside of religion. I know you're wrong first hand.
That's what results from faith based thinking. It allows you to believe false things that are not only insufficiently supported, but as in this case, contradicted by evidence, testimony, and common sense.