The phrase "predominant religion" means the most commonly practiced or widely followed religion in a particular region or among a majority of people in a country or area. Full definition
Just like how people today look at slavery in the United States as unacceptable and barbaric, centuries from now children being educated will read about the once predominant religions in our society and think the same thing. (religion.blogs.cnn.com)
You'd think that Christianity was the predominant religion in America or something — or that fundamentalist Christians were a vociferous, politically and socially influential group. (religion.blogs.cnn.com)
You can't just have Christianity and Judiaism, the predominant religions. (religion.blogs.cnn.com)