One often hears from believers and non-believers alike that worldwide atheism would not make humans better or more moral. That it is our nature to divide ourselves into groups who compete with each other violently. I think there is some truth to this criticism. (I highly recommend reading "The Blank Slate" by Stephen Pinker in which he catalogs the research showing that there is such a thing as innate human nature regarding various topics including this one.)
The weak and excessively needy will always seek to manipulate others to do their bidding, and one of the quickest ways to do that is to try to form one's own little flock of birds of afeather. A group of like minded people will tend to coalesce into a pack separate from the rest, and their very similarity of thought, feeling and psychology will not only make them easier to control, it will make their behavior more extreme because of a lack of dissenting voices.
Religion makes it much easier for the unscrupulous to divide and control others because it divides people into separate groups artificially and allows leaders of those groups to tell the flock what and how to think. It may be impossible to prevent people from dividing themselves into groups, but worldwide atheism would limit the phenomenon by removing dividing lines that are factually nonexistent. If people must divide themselves into groups, let them at least be limited to differences that are based on facts.
The problem, of course with dividing ourselves into groups is that the members of the separate groups then don't feel nearly as much, if any, moral duty toward those in the groups to which they don't belong. This notion, known as "in-group morality" or sometimes as "in-group/out-group morality" is probably one of the most insightful and revealing ideas concerning human behavior and morality to come along. The evidence for this aspect of human behavior is overwhelming, and it's predictive power is absolute. Yet many are reluctant to accept it because of its implications regarding humanity.
I highly recommend reading the paper at this link because it includes not only references to a great deal of scientific research regarding this phenomenon, but also analyzes biblical text to show that this same in-group/out-group morality was inherent in the Abrahamic religions. Contrary to what the members of those religions would have us believe, they are indeed members of hostile groups who feel virtually no moral duty to those who aren't members of their particular group.