J. M. Robertson,
A Short History of Christianity
“ The strength of the papacy as against its many enemies lay (1) in the strifes of States and nations, in which the pope could always intervene; (2) in the feeling of many serious men that a central power was needed to control strife and tyranny; (3) in the compiled system of canon law, which expanded still further the code of the Decretals and of Gratian, and constantly exalted the papal power; (4) in the orders of preaching friars, who acted as papal emissaries, and kept in partial discredit the local clergy everywhere ”
