Over the ages, many things have been written, and many things have been believed. I appreciate that the Bible has many believers, but that by itself is evidence of only one thing: that the Bible has many believers. As noted in a previous answer - if we are not required to reason a view, we can state anything as fact. Thus there is a burden of proof on both sides. Atheism is approaching this burden of proof through scientific method, and it is astonishing the new insights it provides - I don't just mean re-interpreting words that have been written both others : I mean original research and ideas, researched, some found to be useful and iterated, some found to be incorrect and dismissed.
What is right is not always popular and what is popular is not always right.
The default position with any fact based assessment is to look at a combination of factors, including:
- historical evidence in context (which I'll come back to)
(and many other points)
There is nothing verfiable about religious claims. No reliable, repeatable test can be made. We should here limit outselves to things that directly provide evidence for some kind of deity, not simply the "beauty of creation", etc, since that is not causal. The argument "we exist and things are ok, thus God" makes a huge presumption.
It is not reasonable to assume a God. By which, I mean in exactly the same way that if I said "when I drop my pen, invisible pixies push it towards the ground; these magic pixies cannot be detected by any means" - the above would fit with the observation (the pen falling), but:
- there is no requirement for the pixies in the theory
- given the definition (cannot be detected by any means), there is no test that could disprove the theory (this is actually very important in the scientific method)
- the existence or non-existence of the pixies tells us nothing about the universe
Many attempts have been made (over the ages) to prove the existence of a deity through logic; they are all easy to dismiss as abuses of "logic".
This might offend: the Bible is not evidence; the sources are limited and biased, many accounts do not tally with other known records (or even similar accounts in the Bible); it has been extensively edited and re-edited; most of it was written centuries after the supposed accounts; it repeatedly shows even from God, a perspective limited to the knowledge available at the time. All we can prove from the Bible is that: these things have been said and believed.
The modern age
The common interpretation is that the Bible, along with much religion and philosophy, is the approach that relatively primitive cultures have used to explain the awesome fact that we exist. That we exist is amazing, and no attempt is made to devalue that - however, today we have more powerful tools to bring to this. Modern physics, biology, geology, astronomy, etc provide much better explanations for everything in the Bible, that do not require a "leap of faith", but instead have sound reason. Each scientific development erodes our human ignorance, leaving any deity fewer and smaller holes to hide in.
As Carl Sagan wrote:
Extraordinary claims require extraordinary evidence
A claim along religious lines is an extraordinary claim; no religious group has ever produced any compelling evidence for such. So yes: atheism is the reasonable "default position", in the same way that our default position for things falling should not be "pixies".