As an Atheist I find it a bit difficult to think that a year/time system created from religious belief managed to spread across the world. I am not trying to cause religious arguments. I know as well the Roman Empire existed a good bit of time before and after this AD system was formed, and they weren't christian or such so where did that all pop up from?

I'm a bit confused and would appreciate an explanation on how the Anno Domini thing came about and started as the worldwide measurement of years.