Is it possible to be a single religion all over the world?

Unlikely, given that religion is a function and a symptom of human tribalism and will therefore necessarily diverge along cultural, geographical, and historical lines. However, if we were to, in the future, implement a one-world government and dissolve all national borders, it could be possible for religion to be more homogenous.

The only other way it would happen is if a religion arose that was actually demonstrably, empirically true, because then people could test its veracity without appeals to holy texts and revelations, and thus it would become as accepted as the major findings of modern science and medicine. However, seeing as how it's highly unlikely that such a religion will arise, and it's almost entirely impossible that any current religion is even remotely accurate, I doubt this alternative is plausible.