Skillful assumptions

Atheism is not a Noble Truth. Nor is the shape of the Earth.

MN28:2.1: “The footprints of all creatures that walk can fit inside an elephant’s footprint, so an elephant’s footprint is said to be the biggest of them all. In the same way, all skillful qualities can be included in the four noble truths. What four? The noble truths of suffering, the origin of suffering, the cessation of suffering, and the practice that leads to the cessation of suffering.

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?

The through one should foster is that all being are subjected to the 3 marks of existence, the law of cause and effect (Hetu-phala vada), Dependent Origination and the 4 Noble Truths which can be considered skilful. If you believe in a God who is almighty and above these then this is unskilful. If how on to this view tightly and develop greed, hatred and delusion, then this is unskilful.

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

This is neither skilful nor unskilful. Skilful actions are those motivated by non-greed, non-hatred and non-delusion and unskilful are those motivated by greed, hatred and delusion. Unskilful actions are the Wrong part (wrong view ... wrong concentration) of the Noble 8 Fold Path (4the Noble Truth) and skilful action are the Right part (right view ... right concentration) of the Noble 8 Fold Path. If this view was to be contrary to skilful roots (non-greed, non-hatred and non-delusion), 3 marks of existence, the law of cause and effect (Hetu-phala vada), Dependent Origination and the 4 Noble Truths this will be unskilful and if it was in accordance to skilful roots, 3 marks of existence, the law of cause and effect (Hetu-phala vada), Dependent Origination and the 4 Noble Truths it will be skilful. But the idea that the earth is flat does not confirm to wither.

Also see: The Dhamma in Buddhism part 4

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.

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.