user151: Does atheism in Buddhism mean you let a belief arise that God doesn't exist? Is this a skillful assumption? Why or why not?
Was it right view for one to believe that the world wasn't round during the time when it seemed to everyone that the Earth was flat? Was a belief in a flat Earth a skillful assumption? Why or why not?

user19257: Atheism is described with conflicting ideas in many different ways. However, it is generally pointing towards not believing in gods / God.
Buddhism is full of information about gods. However, it clearly says that there is no creator. That means everything in the universe, including gods are not created by anyone.
Now, we have a question about Buddhism if it is really a “religion” for an atheist.
For me, if you believe in something firmly that means except as an hypothesis, you are not an atheist.
If I say a long story short, Buddhism is indeed a “religion” for the atheists. 
Why? Buddha has clearly shown the way to discover the reality of things without any doubts.
That is path to enlightenment through Jana. In the 4th Jhana you will have the opportunity to dive deep in to the reality which is normally hidden from the surface view. In the 4th Jhana, you will Be able to uncover 2 or more special Knowledge/ sense bases which will eventually bring you to that realization.
There are so many distractions, rituals and other ideas surrounding the core knowledge which is hidden from the average follower. 
Therefore, clearly an atheist can be a Buddhist however, until that person reach the final goal, he or she should carry some sort of assumptions.

user471: 
OP: Does atheism in Buddhism mean you let a belief arise that God doesn't
  exist? Is this a skillful assumption? Why or why not?

To put your "salvation" or your liberation from suffering into someone else's hands, is unskillful in Buddhism.
To reinforce belief in a self, is unskillful in Buddhism.
To take anything that is impermanent as permanent, is unskillful in Buddhism.
Faith in an eternal permanent Supreme God who is one's personal saviour would include all three unskillful views above, in some way or the other.

OP: Was it right view for one to believe that the world wasn't round
  during the time when it seemed to everyone that the Earth was flat?
  Was a belief in a flat Earth a skillful assumption? Why or why not?

Whether the Earth is round or flat is not of concern to Buddhism, and can be left to science. In this case, it is skillful to rely on scientific methods.
However, metaphysical investigations that cannot be proven by science is not skillful, as seen in the Acintita Sutta:

"Conjecture about [the origin, etc., of] the world is an
  unconjecturable that is not to be conjectured about, that would bring
  madness & vexation to anyone who conjectured about it.


151: <p>Does atheism in Buddhism mean you let a belief arise that God doesn't exist? Is this a skillful assumption? Why or why not?</p>

<p>Was it right view for one to believe that the world wasn't round during the time when it seemed to everyone that the Earth was flat? Was a belief in a flat Earth a skillful assumption? Why or why not?</p>
 19257: <p>Atheism is described with conflicting ideas in many different ways. However, it is generally pointing towards not believing in gods / God.
Buddhism is full of information about gods. However, it clearly says that there is no creator. That means everything in the universe, including gods are not created by anyone.
Now, we have a question about Buddhism if it is really a “religion” for an atheist.
For me, if you believe in something firmly that means except as an hypothesis, you are not an atheist.</p>

<p>If I say a long story short, Buddhism is indeed a “religion” for the atheists. 
Why? Buddha has clearly shown the way to discover the reality of things without any doubts.
That is path to enlightenment through Jana. In the 4th Jhana you will have the opportunity to dive deep in to the reality which is normally hidden from the surface view. In the 4th Jhana, you will Be able to uncover 2 or more special Knowledge/ sense bases which will eventually bring you to that realization.
There are so many distractions, rituals and other ideas surrounding the core knowledge which is hidden from the average follower. </p>

<p>Therefore, clearly an atheist can be a Buddhist however, until that person reach the final goal, he or she should carry some sort of assumptions.</p>
 471: <blockquote>
  <p><strong>OP:</strong> Does atheism in Buddhism mean you let a belief arise that God doesn't
  exist? Is this a skillful assumption? Why or why not?</p>
</blockquote>

<p>To put your "salvation" or your liberation from suffering into someone else's hands, is unskillful in Buddhism.</p>

<p>To reinforce belief in a self, is unskillful in Buddhism.</p>

<p>To take anything that is impermanent as permanent, is unskillful in Buddhism.</p>

<p>Faith in an eternal permanent Supreme God who is one's personal saviour would include all three unskillful views above, in some way or the other.</p>

<blockquote>
  <p><strong>OP:</strong> Was it right view for one to believe that the world wasn't round
  during the time when it seemed to everyone that the Earth was flat?
  Was a belief in a flat Earth a skillful assumption? Why or why not?</p>
</blockquote>

<p>Whether the Earth is round or flat is not of concern to Buddhism, and can be left to science. In this case, it is skillful to rely on scientific methods.</p>

<p>However, metaphysical investigations that cannot be proven by science is not skillful, as seen in the <a href="https://www.accesstoinsight.org/tipitaka/an/an04/an04.077.than.html" rel="nofollow noreferrer">Acintita Sutta</a>:</p>

<blockquote>
  <p>"Conjecture about [the origin, etc., of] the world is an
  unconjecturable that is not to be conjectured about, that would bring
  madness &amp; vexation to anyone who conjectured about it.</p>
</blockquote>
