There are over 2 billion Christians in the world, how can all those people be wrong?

Quite easily actually. Until new information is available the whole world may be wrong about a certain idea. Like the shape of the world or the center of our solar system for example. It's just that religion is a faith based concept and can still exist even with tons of new information at our disposal. 

These faith based concepts that can be neither proved nor disproved are not all that uncommon. Take ghosts for example. There was a time when a grown man could flee his home in fear of a haunting without embarrassment. It would be a rational and safe thing to do, to get out of the house. These days? Though there are still a large amount of people who believe in ghosts, he would still be laughed at and be plastered over the internet as a joke. So what happened? Over time with the lack of scientific data or believable sightings people eventually grew out of that fable.

Unfortunately religion is just something that stuck. There could be a number of reasons why this has happened. Probably the best reason is because religion can be talked about in every day life. If something good happens people might thank god for that gift or miracle, whereas it's pretty difficult to keep mentioning ghosts for the reason behind everything. This could be why that belief has faded more quickly than religion despite there being an equal amount of scientific proof for both ideas. However, religion indeed is fading out.

The polls on this site are interesting. http://digitaljournal.com/article/267117 It mostly talks about evolution, yet it is still worth mentioning as the relationship between the two ideas are so intimate. The numbers clearly show that religion is an older idea, and that newer generations are starting to call BS. Those who go to school and get an education are getting the idea that evolution is pretty hard to argue, while those who do not graduate high school are probably more likely to be the ones who attend church regularly. (The poll does not show that last part before people start responding about that comment.)

In summary, two billion people can be wrong. It has happened. People try to explain things with supernatural explanations. That answer stands as true until a "nerd" uses science to find a real answer, then the supernatural explanation is soon faded from memory and laughed about at younger generations. The same will be true for religion; there will be a time when religion is minority. Like the pockets of people who believe the moon landing was a fake, in the future religious people will rightfully be looked at as crazy.