in partibus infidelium

Latin phrase

in par·​ti·​bus in·​fi·​de·​li·​um in-ˈpär-ti-ˌbu̇s-ˌin-fi-ˈdā-lē-ˌu̇m 

: in the regions of the infidels

—used in a bishop's title after the name of a diocese conquered by a power of another faith