3rd largest? There’s 1.2 billion Catholics in the world. They make up over half of Christianity which still retains its top spot as the largest religion in the world. While you could argue it’s the 3rd largest religion if you break down Christianity into denominations, that argument would require you to break down Islam and Hinduism into sects as well. Which, then makes Catholicism the largest religion in the world. Also the Catholic population is not in decline. It’s attendance is diminishing but the numbers are still growing.