Religion and Culture

In much of science fiction, especially stories that are told in the distant future and contain multiple alien cultures, the primary unit of social organization is the planet and not the nation. In these visions of the future this results in an entire planet becoming a cohesive culture which includes a more or less unified planetary religion. Likewise there is little variation amongst a particular alien race, all aliens from planet X think, behave, and believe the same way. I wonder if this is plausible. Just based on our own experience it seems like it is not possible to unite an entire planet under one shared culture. However, globalization has already spread ideas and material culture around the world so that we have much more in common with a person on the other side of the globe than we would have in the past. It seems likely that this trend will continue and that this process would be hastened if we suddenly found ourselves as one race among many. We might start to think of ourselves as being from planet Earth instead of from the United States, Brazil, or Turkey. Supposing this happens what would be the effect on religion? Would religions meld together to create something completely different, would we have fierce religious battles, or maybe religion would simply fade even more into the background, making it an even more private affair than it already is.