A recent study has found strong correlations between how strong a country's belief in Christianity is and how high their levels of violence and teen pregnancy are. (The survey wasn't about Christianity, per se, but it focused on mostly Christian and formerly Christian nations, with the exception of Japan). One result from the study:
A few hundred years ago rates of homicide were astronomical in Christian Europe and the American colonies (Beeghley; R. Lane). In all secular developed democracies a centuries long-term trend has seen homicide rates drop to historical lows (Figure 2). The especially low rates in the more Catholic European states are statistical noise due to yearly fluctuations incidental to this sample, and are not consistently present in other similar tabulations (Barcley and Tavares). Despite a significant decline from a recent peak in the 1980s (Rosenfeld), the U.S. is the only prosperous democracy that retains high homicide rates, making it a strong outlier in this regard (Beeghley; Doyle, 2000). Similarly, theistic Portugal also has rates of homicides well above the secular developed democracy norm.
And for those who find reading studies boring, you can read this somewhat opinionated summary.
It is possible that part of the problem here is that those who "believe" often look down on those who don't. I and many of my friends back in the US can readily testify to this. It should go without saying that when you have a self-righteous group looking down on those who don't share their views, societal tension is a natural result. But if you're tempted to respond to this, read the study first, regardless of whether or not you agree with me.
Update: if this stuff intrigues you, you have to read the reply by chromatic. chromatic and I have differing viewpoints on this topic and makes a brilliant response to this. He and I disagree a lot about these issues, but I can't help but respect his point of view.