I do not believe in any god, spiritual being, existence of any higher authority so maybe I shouldn't be answering this question, but I will because I am looking for excuses not to do my assignment.
I like to think that I wouldn't behave any differently as my ethical/moral framework would be the same, but then would it?
Has there been any human society where there hasn't been an influence of some sort of god/spiritual being that has the potential to cause problems for humans and therefore needs to be placated/feared. Even if it is a tree spirit that might wreak some terrible revenge for not being sufficiently revered or because you cut a branch, somehow humans have explained the world through the influence of a god like figure. Therefore we have modified our behaviours in order to keep in with a god, salvation and damnation. Now whilst this is becoming less influential on how we behave (materialism of the 80's), it still does have an impact and shape the culture we are immersed in.
So, in your parallel world, that has never been influenced by any type of religion, would our morality and ethical framework be the same, better or worse?