cujus regio, ejus religio

Latin phrase

cuj·​us re·​gio, ej·​us re·​li·​gio ˌkü-yu̇s-ˈre-gē-ˌō ˌe-yu̇s-re-ˈli-gē-ˌō 

: whose region, his/her religion : subjects must accept the religion of their ruler