So, how do I think each group would handle this event and what pros and cons do you think each group would face?
Some of the atheists, as I say below, would be possible to choose to start believing in a religion, after the incident, but most of them, as far as I am concerned, would not consider it 'an act of God', and would simply regard it as an illusion, or something inexplicable, non - religion related.
In my opinion, the atheists would definitely be at an advantage; smaller population means fewer needs, there would be no disputes about religion similar to the ones today in size (even though a number of the atheists could decide to start 'having faith', after that event, perhaps the majority would still remain the same or, in an extreme case, they could split into two or three groups - true atheists and atheists who formed their new religion(s)/chose one or some of the old ones and went with it/them, either unchanged or modified), science would advance, most probably, even more rapidly. Life would have little reason not to revert to normal relatively soon.
Morals could, however, take a hit, as Eless mentioned, along with tradition (considering most of the people to keep traditions alive tend to be religious, as well). I don't believe, though, that the hit (to morals, at least) would be that hard or one they would not be able to easily recover from, since society (in its structuring) itself should not change that much. Traditions and customs could be lost, on the other hand, and thus the nations could shift and change, since their identities could be altered. Not to mention some countries would easily almost completely vanish from the 'atheist Earth'.
The way I see it, the theists would be in a whole lot of trouble. Each religion group would try to explain the event in its own way, and, of course, convince the others that they are correct. Even if war and chaos would not almost instantly take over, it would be bound to happen in the near future. And of course, the church would become incredibly important, again, since many would easily become fanatics, affected by what happened, most probably believing in their religion more than ever, in a fashion that could harm others as well as themselves.
If we looked at a map of Earth where each area would be coloured to be correspondent to what the dominant religion is, atheism would be white, let's say, right? It is easy to see that once that 'white matter' is out of the way, there is nothing separating one religion group from another; this event would cause quite some clashes between stronger (as in 'denser' groups and weaker (not as common in an area) ones in every day life for many people, other than the potential wars to be brought about.
Other than all that, science would undoubtedly fall way behind as a priority. Even the few religious scientists to be left would most probably be under great pressure by the public to stop researching or experimenting or doing pretty much anything, since it could be considered what got them there in the first place.
I can honestly think of no significant pros they would have, other than the fact the bonds between people of the same religion would strengthen, with societies to have a 95%+ of one religion group functioning better.
In general, I think the theists' Earth would be turned into what it was half a century, or perhaps a century and even more ago, with the way back being extremely difficult to walk, even without considering the potential wars to break out.
Well, I think this is my first WEPR post. Was it satisfactory to read and did it contribute to the conversation?