6 Here, however, it will be fitting to recall the mutual relations of the spiritual and of the temporal order, for many minds on this matter fall into a two-fold error. There are some, for instance, who are not satisfied with distinguishing between politics and religion but separate and completely isolate the one from the other; they wish them to have nothing in common, and imagine that the one should exercise no influence over the other. Such men, in truth, differ but little from those who desire the exclusion of God, the Creator and Sovereign of all things, from the constitution and administration of the State; and the error they profess is the more pernicious that they thereby rashly debar the State from its most abundant source of prosperity. The moment religion is removed, those principles are of necessity shaken on which the public welfare most of all rests, and which drive their greatest force from religion, among the first of which are government with justice and moderation, obedience from a sense of duty, the submission of the passions to the yoke of virtue, to render to each his due, to leave untouched that which is another's.
Source: Cum Multa (Vatican.va)