Atheists tend to describe atheism as the most reasonable and rational position to hold. But then again I'm sure most people believe that of whatever worldview they hold to, so I wouldn't hold that against them.
The short explanation is that they believe science is on their side and the evidence for theism is wanting. I will here clarify that by 'atheist' and 'atheism' I refer to the views that "God does not exist" or "God probably does not exist".
This doesn't speak to the agnostic non-theist who is on the fence.
But consider this worldview. Does it stand up? Can it stand up? Maybe. But it is on the thinnest of ice.
First of all consider that the reason atheists reject theism is that they believe there is either no evidence for it, or the evidence is weak. Well this is an obvious double-edged sword. There is no evidence for the claim "There is no God". Most atheists I have asked refuse to defend that statement and even refuse to commit to it. I hope it is because they realise that they can't.
The best arguments in the atheist arsenal are generally regarded as the Problem of Evil and Divine Hiddenness. Neither of these conclude "There is no God". They at best only comment on a specific definition of God. So even if they were good, successful arguments, they could not carry atheism.
Already anyone who thinks of themself as an atheist should be more comfortable in the agnostic non-theist camp.
The second thing to consider is scientific. It is often said that the theory of Evolution gives people intellectual permission to be atheists. It seems a great many atheists hang their worldview on this.
Should they be so sure of the link between evolution and atheism? A large number of believers are theistic evolutionists, explaining that there is no contradiction between God's creation and the mechanics of evolution and natural selection. If that is the case, then atheists can not hang their worldview on this theory.
Alvin Plantinga and others go further with the Evolutionary Argument Against Naturalism. Their argument concludes that if Evolution did happen, it would require a miracle. If that is the case, then Evolution is actually proof against atheism!
On the other hand, Intelligent Design defenders point out that there is a difference between the indisputable facts of micro-evolution and the lack of evidence for macro-evolution and common descent. If macro-evolution did not happen, then atheists lose their intellectual permission to hold their view.
The point being that if macro-evolution is true, there are good reasons to think that means atheism is false, and if macro-evolution is false, it is almost certain that atheism is also false.
The weakness of evidence also applies to abiogenesis. Although this is a separate theory to evolution, it is one that atheists need for their worldview to be true.
If both atheists and theists can believe in evolution, abiogenesis makes a stronger case for the truth of atheism. Essentially the first cause of evolution must be atheistic abiogenesis or theistic genesis.
Considering the utter lack of reason to believe in abiogenesis, and the numerous reasons to believe in creation, atheism doesn't hold up well evidentially at all.
The third thing we will consider may best be what shows atheism to be on incredibly weak ground. Atheism rejects the supernatural. It rejects the soul, miracles, spiritual experiences, answered prayers, near death experiences, and so on.
Think of how many people have claimed to have had an out of body experience, or seen a miraculous healing, or have been haunted by a demon, or have had a prayer heard and responded to. The number is surely in the billions.
If only one of these claims is true, then atheism is false. Only one supernatural event needs to have happened for the worldview of non-belief to be shattered. The odds are not in their favour.
You can also add to this how atheism struggles to account for things such as objective morality, free will and consciousness, and experiences of beauty and tastes. All these things fit comfortably in a theistic worldview. There are no comparable problems.
As previously said, much of this will not apply to the agnostic non-theists. But then to those on the fence, a consideration of the weakness of atheism should at least be something to take into account.
