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.
If indeed, although Christianity is the one that presents the highest percentage, they are also the religion most detached from the customs that they should practice because of their religion