I think most people will agree that christians are the biggest religious group.But its hard to tell how big the exact amount of christians on the world is.
I think it will be around 30% or so.

It would be interesting to know how many of this people really believe in the christian good. A lot of people are just christians on the paper but don't go to church , pray or do other things. I wouldn't really count them as christians ( this people exist for all religions)
In a few years probably atheists will be the biggest religious group.