The challenge is to have a situation where one religion is dominant over the whole world. There can be different sects, but no more different than the differences between the sects of Christianity or Islam. The religion doesn't need to have literally every person as an adherent, and there can be atheists or people of other, minor religions. It does, however, have to be dominant over most of the world and have a majority of the world's population as believers.
The only pre-1900 scenario I can think of is the ultimate triumph of loose paganism in the West, as well as the East and Center. A sort of animism is, while very different from region to region, accepted as all one huge "spirituality," though very few have a strict, codified religion in the Western sense.
After 1900, and one large-scale nuclear war later and Christianity would fit the bill.
After 1900, and one large-scale nuclear war later and Christianity would fit the bill.