Complex sentence: The first bull was issued in about 1120 by Calixtus II and was intended to protect Jews who suffered during the First Crusade, during which over five thousand Jews were slaughtered in Europe.
Split into simpler sentences: The first bull was issued in about 1120 by Calixtus II and was intended to protect Jews. It was prompted by the First Crusade, during which over five thousand Jews were slaughtered in Europe.