2 And yet a hackneyed reproach of old date is levelled against her, that the Church is opposed to the rightful aims of the civil government, and is wholly unable to afford help in spreading that welfare and progress which justly and naturally are sought after by every well-regulated State. From the very beginning Christians were harassed by slanderous accusations of this nature, and on that account were held up to hatred and execration, for being (so they were called) enemies of the Empire. The Christian religion was moreover commonly charged with being the cause of the calamities that so frequently befell the State, whereas, in very truth, just punishment was being awarded to guilty nations by an avenging God. This odious calumny, with most valid reason, nerved the genius and sharpened the pen of St. Augustine, who, notably in his treatise, The City of God, set forth in so bright a light the worth of Christian wisdom in its relation to the public wealth that he seems not merely to have pleaded the cause of the Christians of his day, but to have refuted for all future times impeachments so grossly contrary to truth. The wicked proneness, however, to levy like charges and accusations has not been lulled to rest. Many, indeed, are they who have tried to work out a plan of civil society based on doctrines other than those approved by the Catholic Church. Nay, in these latter days a novel conception of law has begun here and there to gain increase and influence, the outcome, as it is maintained, of an age arrived at full stature, and the result of progressive liberty. But, though endeavours of various kinds have been ventured on, it is clear that no better mode has been devised for the building up and ruling the State than that which is the necessary growth of the teachings of the Gospel. We deem it, therefore, of the highest moment, and a strict duty of Our apostolic office, to contrast with the lessons taught by Christ the novel theories now advanced touching the State. By this means We cherish hope that the bright shining of the truth may scatter the mists of error and doubt, so that one and all may see clearly the imperious law of life which they are bound to follow and obey.
Source: Immortale Dei (Vatican.va)