Is nihilism the logical extreme of atheism?

[Neo-Scholastic]

Just to be clear, my definition of nihilism is very broad: holding that all values are baseless and that nothing can be known or communicated.

... I fail to see how you can reconcile a requirement that nihilism invalidates reason and knowledge with a position which extends the hope that the universe is knowable.

People cannot help but to assign value to the things in their lives. The nature of the OP question is whether or not someone’s philosophical position, in this case atheism, is inconsistent with the very human ability to find value. Thus it doesn’t matter whether the person has hope; but rather, what matters is if that hope is justified, i.e. does their philosophy fulfill its promise.

Also, your division between atheist and theist ...doesn’t account for the entire pie that is atheism. Yes, many atheists deny the supernatural, but then there are atheists who ...hold metaphysical views that aren’t committed to a thoroughgoing naturalism. ...the number of atheists of whom your views are descriptive is entirely too narrow to be considered the essence of atheism...

There is some truth to your critique. I have generalized my writing to address the most broadly understood division within the Western tradition. Without being explicitly stated, that appears to have been the context of the OP question. Outside the Western tradition the atheist/theist distinction ceases to be a useful way of categorizing the metaphysical and cosmological belief systems such as you described: African animism, Buddhism, etc.

... not all people’s views are that well thought out, nor need they be for them to be complete atheists...It’s possible to have a sense of meaning without having an understanding of where that meaning comes from...Not knowing is as much a part of atheism as knowing.

Correct, but I was very clear in stating that they could not justify saying they have a raison d’etre in the philosophical sense. Such a person is in the same position as a “blind faith” believer, one that cannot give an account for their doctrines.

Purpose, lasting value, and significance. These three seem somewhat arbitrarily drawn...

To avoid arguments over definitions, I choice to address those concepts that I believe minimally address nihilism as I defined it. There could be others

Sartre’s philosophy rested on ... we do in the present. What we have been and intend to become may give our lives purpose and imbue each act with meaning and significance, even if only to us.

Having mentioned Sartre, you have moved the conversation away from the analytic tradition and towards the issues of continental philosophy, in which cultural and personal experience, rather than universal grounds, serve as the basis for interpretation of the human condition. This is a fair move and one that I welcome, since my post on the nature of significance comes largely from my understanding of post-structuralism. I think it reasonable to interpret phrases like “if only for us,” that reference arbitrary sources value as falling within my definition of nihilism as “the doctrine that all values are baseless.” The general objection has been that people find their own meaning, as in...

...I don’t see why meaning has to be, nor that non-transcendent meaning is of necessity nihilistic. Temporary pleasures are still pleasures. Momentary setbacks are still setbacks. Why does meaning have to be grounded by relationships to something external and eternal for it to be real? Why does ephemeral meaning simply not count, while transcendent meaning is the only kind that does?

By making meaning a matter of personal preference and cultural whim, people remove again alienate signification from the larger reality. Once the relation between signfiers and the signified has been severed, then multiple interpreters have no consistent basis by which they can relate. The key to avoiding nihilism (in the Western sense) is to identify absolutes that apply to and govern the relationships between multiple knowing subjects. This is a project that self-referential systems cannot logically undertake.

Having said all this, I will let you have the last word. Unlike, Whateverist, I have tired of the thread and must return to more pressing problems.