45 It follows clearly, therefore, that Catholics have just reasons for taking part in the conduct of public affairs. For in so doing they assume not nor should they assume the responsibility of approving what is blameworthy in the actual methods of government, but seek to turn these very methods, so far as is possible, to the genuine and true public good, and to use their best endeavours at the same time to infuse, as it were, into all the veins of the State the healthy sap and blood of Christian wisdom and virtue. The morals and ambitions of the heathens differed widely from those of the Gospel, yet Christians were to be seen living undefiled everywhere in the midst of pagan superstition, and, while always true to themselves, coming to the front boldly wherever an opening was presented. Models of loyalty to their rulers, submissive, so far as was permitted, to the sovereign power, they shed around them on every side a halo of sanctity; they strove to be helpful to their brethren, and to attract others to the wisdom of Jesus Christ, yet were bravely ready to withdraw from public life, nay, even to lay down their life, if they could not without loss of virtue retain honours, dignities, and offices. For this reason, Christian ways and manners speedily found their way not only into private houses but into the camp, the senate, and even into the imperial palaces. "We are but of yesterday," wrote Tertullian, "yet we swarm in all your institutions, we crowd your cities, islands, villages, towns, assemblies, the army itself, your wards and corporations, the palace, the senate, and the law courts."(26) So that the Christian faith, when once it became lawful to make public profession of the Gospel, appeared in most of the cities of Europe, not like an infant crying in its cradle, but already grown up and full of vigour.
Source: Immortale Dei (Vatican.va)