But from the time when the civil society of men, raised from the ruins of the Roman Empire, gave hope of its future Christian greatness, the Roman Pontiffs, by the institution of the Holy Empire, consecrated the political power in a wonderful manner. Greatly, indeed, was the authority of rulers ennobled; and it is not to be doubted that what was then instituted would always have been a very great gain, both to ecclesiastical and civil society, if princes and peoples had ever looked to the same object as the Church. And, indeed, tranquility and a sufficient prosperity lasted so long as there was a friendly agreement between these two powers. If the people were turbulent, the Church was at once the mediator for peace. Recalling all to their duty, she subdued the more lawless passions partly by kindness and partly by authority. So, if, in ruling, princes erred in their government, she went to them and, putting before them the rights, needs, and lawful wants of their people, urged them to equity, mercy, and kindness. Whence it was often brought about that the dangers of civil wars and popular tumults were stayed.
–Diuturnum Illud (On the Origin of Civil Power)- Encyclical of Pope Leo XIII