The Messiah concept is fundamental in Christianity. Originally an earthly king ruling by divine appointment ("the anointed one", as the title Messiah had it), the "son of David" became in the last two centuries BCE the apocalyptic and heavenly one who would deliver Israel and usher in a new kingdom. This was the background to the concept of Messiahship in early Christianity, which interpreted the career of Jesus "by means of the titles and functions assigned to David in the mysticism of the Zion cult, in which he served as priest-king and in which he was the mediator between God and man".