The fact that there are two billion or so Christians in the world today is largely due to one guy: Constantine the Great. He's the Roman emperor who legalized Christianity, converted to Christianity himself and cleared the path for spreading the religion throughout Europe and then across the globe. So go for it, atheists. Here's who you have to blame.

But Constantine wasn't always a Christian. In 310 A.D., not-yet-emperor-Constantine was fighting a civil war against another guy laying claim to the throne, Maxentius, who we believe made such claim based purely on the fact that his name was awesome.
After several months of fighting, Constantine and Maxentius met for a single, decisive battle. Maxentius had almost twice as many men but, as it turns out, Constantine had God on his side. A few hours before the battle, Constantine "saw with his own eyes in the heavens a trophy of the cross arising from the light of the sun, carrying the message, 'Conquer By This.'"

And he did.
The World-shattering Coincidence
Depending on how religious the person is you are talking to, the sign he saw in the sky was either a miracle or some ridiculous thing he imagined/made up later for the awesomeness. Turns out, it was apparently just a massive, flaming meteorite that just happened to be flying by at that moment.
That's right, the entire religious makeup of the modern world turned on a random hunk of space rock.
OK, that doesn't explain the words "Conquer By This" flying across the sky. It's possible here that he decided to enhance his meteor shower viewing by dropping some nice acid.
If however it turns out the meteor's trajectory was such that its flame trail did somehow spell out "Conquer By This" across the sky in the man's own language, well, we probably have to give props to the divine intervention school of thought.

And How Did it Change The World?
Constantine went on to win and become Roman Emperor. Forever altered by the events of the battle, he passed the Edict of Milan, which granted religious tolerance to everyone!
Christianity exploded in popularity and a few decades later, was declared the sole religion of the empire. And the rest is... well, you know.