Is nihilism the logical extreme of atheism?

[Esquilax]

Clearly, the term nihilism does encompass a large number of related concepts. Thus I do not think a single line of reasoning leads from atheism (as simply a lack of belief in God or gods) to nihilism, moral or otherwise. As Genkaus correctly points out, failure to find a solution to nihilism doesn’t make it false, which would be an argument from ignorance. At the same time, no one cannot justify saying they have a raison d’etre (if it matters to them) without having some way to ground the meaning of their life with three basic concepts: purpose, lasting value and significance. As I see it, atheism undermines all three. And without that solid foundation, all atheists are tacit nihilists no matter how adamantly they deny it.

Bolding mine. How does that follow? And how are you defining your three concepts, more importantly? Why is a self-defined purpose insufficient? How long does value need to last before it crosses the threshold of legitimacy, and why? How significant does a raison d'etre need to be before it becomes acceptable to you?

Moreover, what is it about a god that resolves these issues? This seems to me a total non-sequitur: why must we gain those things from outside of humanity for them to be worth enough?

To me, the truly honest atheist is one that accepts existential absurdity. When I was an atheist, I found myself able to counter the occasional moments of despair with a pleasing noble defiance of my fate, that “rage against that dark night”; the myth of Sisyphus; Zarathustra’s dancing; and all that sort of heady stuff. But there is nothing wrong with simply focusing on the mundane, just getting on with getting on, and “enjoying the ride.”

This all seems needlessly melodramatic, to me. If you want to characterize the universe sans-god as something pointless and empty then that's on you, but personally I find plenty to be fascinated with in what we've got, nor do I find unlocking the mysteries of the universe to be a "mundane" task for a species merely because no magic was involved.

One of the actual joys of atheism is defining your own purpose in life. Such joy is an emotional response that doesn’t rationally counter nihilism. When someone defines purpose as that outcome towards which something is directed, then they are invoking Final Cause. Atheism, per se, does not exclude final causes, but the reduction of the world to purely efficient causes acting on material bodies does. Therefore ‘purposes’ are illusions born of viewing higher-order processes that are fully determined at lower levels of order. So while it would appear as-if intelligent agents have goals, in actuality there are no final ends and it is irrational to speak about any life having purpose.

Why the false dichotomy? Since when was externally derived, ultimate purpose the only legitimate form of it?

You're beginning with a premise that I don't buy, that if life is solely composed of material causes then matter is all there is. But that's a fallacy of composition, presuming that the whole must function exactly like its constituent parts. Why can't minds capable of deriving purpose arise from material bodies without being completely slaved to them? What makes you think that the base dictates everything that is extrapolated from it?

When people talk about a life’s purpose they usually are thinking of a higher criteria that just goal-seeking behavior and final ends. What they really mean is that their life counted from something, i.e. their life has, or will have, lasting value. Value requires that someone appreciates and desires something which is valued. The value of life for the person living it, seems self-evident, since all other valuables require already having a life. But because life ends, the lasting value of a person’s life depends on their life having continuing value to those remain alive and future generations. Then the sun blows up and with it any value our lives once had. Thus at a bare minimum, for human life to have value there must be some enduring agent to whom human life is valuable.

I wasn't aware that "temporary" and "nonexistent" were synonyms.

Just because something ends doesn't mean it has no legitimate worth. If you actually believed that you'd never see movies, or read books, or even talk to people. After all, you'll eventually forget things about them, therefore diminishing their value, right?