In this book, religion means a group which has a hierarchical power structure leading up to an invisible God. Naturally the God favors the religion which worships Him. Most religions are secret societies. They have an agenda which involves controlling their devotees, all their charity-work notwithstanding. Historically, people seek out secret societies and religions and join them to gain freedom from pain and turmoil.
People also join because they are forced to.
For example, the country they live in leaves no choice. Such was the case in medieval Europe, where the Roman Catholic Church held the wand of dominance. But there is another factor. The popular term for it these days is mind control. It used to be brainwashing, hypnotism.