The phrase "great divider" refers to someone or something that causes division or separation among people or groups. It suggests that this person or thing creates differences and conflicts instead of bringing people together. Full definition
Because he worked with non-fundamentalists, Graham was «the greatest divider of the Church of Christ in the twentieth century.» (religion-online.org)
There's nothing you can defend because religion is the greatest divider there is and has caused more death and destruction than any cause known. (religion.blogs.cnn.com)