
Improper use. In the 21st century, religion shouldn't be priority. Infact, a lot of terrorism and fighting has been because of religion. Heck, mass genocide because of religion (Not religions fault, but if religion hadn't of been around, the Jews wouldn't of been killed)
The crusades. Huge war over holy land. Without religion, to be frank, the world would be a better place. Religion does provide culture, but everything has it's dark side. Like Science can be used to murder and maim.