Why is Christianity so Popular?

This is not a naive question; it is actually quite profound when you think of it.

There are 2.1 billion estimated people who would claim adherence to Christianity in the world. The runners-up of note are Islam at 1.5 billion, secularists at 1.1 billion, Hinduism at 900 million, Buddhism with 376 million, Sikhism (the original Chrislam) at 23 million, and the children of Israel coming in with a poky 14 million. I'd be really curious to see if these statistics changed greatly as of the most recent census.

Wow, looks like Christianity's really popular! And with a world population somewhere between 6 and 7 billion (probably much closer to 7), that means Jesus' got a 30% approval rating! That's better than President Obama's rating, and probably most other presidents (not going to bother looking up that statistic).

So why is the world so messed up? You'd think two vs. 5 would be pretty good odds, especially when not all nonChristians are actively causing chaos. We're all made in God's image, after all, so one would expect that even those who reject God would behave somewhat morally, having The Law written on their hearts (Romans 2:14-15).

Then whence cometh evil?