A guardian angel is a type of angel that is assigned to protect and guide a particular person, group or nation. Belief in tutelary beings can be traced throughout all antiquity. The idea of angels that guard over people played a major role in Ancient Judaism. In Christianity, the hierarchy of angels was extensively developed in the 5th century by Pseudo-Dionysius the Areopagite. The theology of angels and tutelary spirits has undergone many changes since the 5th century. The belief is that guardian angels serve to protect whichever person God assigns them to. The Memorial of the Holy Guardian Angels is celebrated on 2 October.