18 But the Church has always so acted that the Christian form of civil government may not dwell in the minds of men, but that it may be exhibited also in the life and habits of nations. As long as there were at the helm of the States pagan emperors, who were prevented by superstition from rising to that form of imperial government which We have sketched, she studied how to instill into the minds of subjects, immediately on their embracing the Christian institutions, the teaching that they must be desirous of bringing their lives into conformity with them. Therefore, the pastors of souls, after the example of the Apostle Paul, were accustomed to teach the people with the utmost care and diligence "to be subject to princes and powers, to obey at a word,"(22) and to pray God for all men and particularly "for kings and all that are in a high station: for this is good and acceptable in the sight of God our Saviour."(23) And the Christians of old left the most striking proofs of this; for, when they were harassed in a very unjust and cruel way by pagan emperors, they nevertheless at no time omitted to conduct themselves obediently and submissively, so that, in fact, they seemed to vie with each other: those in cruelty, and these in obedience.
Source: Diuturnum (Vatican.va)