‘The royal and the priestly dignity … are bound together in the Christian people by a reciprocal treaty. Each must make use of the other … For the king is girded with the sword so that he may be armed to resist the enemies of the Church and the priest devotes himself to prayers and vigils so as to make God well-disposed towards king and people.’ So Cardinal Peter Damian of Ostia described the mutual dependence of the ecclesiastical and the secular power in his letter of 1065 to Henry IV, exhorting the king to defend the Roman church against the attacks of the antipope. He had stressed the same theme three years earlier, when exhorting the German court to recognise Alexander II as lawful pope. This theory derived from the teaching of Pope Gelasius I (492–6) concerning ‘the sacred authority of bishops and the royal power’. ‘Christ … separated the offices of both powers according to their proper activities and their special dignities … so that Christian emperors would have need of bishops in order to attain eternal life and bishops would have recourse to imperial direction in the conduct of imperial affairs.’ This Gelasian idea remained the basis of the papacy's political thought at the beginning of the pontificate of Gregory VII.