36 Yet, no one doubts that Jesus Christ, the Founder of the Church, willed her sacred power to be distinct from the civil power, and each power to be free and unshackled in its own sphere: with this condition, however - a condition good for both, and of advantage to all men - that union and concord should be maintained between them; and that on those questions which are, though in different ways, of common right and authority, the power to which secular matters have been entrusted should happily and becomingly depend on the other power which has in its charge the interests of heaven. In such arrangement and harmony is found not only the best line of action for each power, but also the most opportune and efficacious method of helping men in all that pertains to their life here, and to their hope of salvation hereafter. For, as We have shown in former encyclical letters,(56) the intellect of man is greatly ennobled by the Christian faith, and made better able to shun and banish all error, while faith borrows in turn no little help from the intellect; and in like manner, when the civil power is on friendly terms with the sacred authority of the Church, there accrues to both a great increase of usefulness. The dignity of the one is exalted, and so long as religion is its guide it will never rule unjustly; while the other receives help of protection and defence for the public good of the faithful.
Source: Arcanum Divinae (Vatican.va)