A coreligionist is someone who belongs to the same religion as you. If you are Muslim, another Muslim is your coreligionist; if you are Buddhist, another Buddhist is. The word does not pick a religion. It is a rather formal, bookish word. You will see it in news and essays more than you will hear it at home. In ordinary talk, people say someone of the same faith, or, inside one church, a fellow believer. Coreligionist is handy when a writer wants one cool word that works for any religion, not only for Christians. It does not mean you like the person, or that you pray in the same building. It only means you share the religion.