4 Next, as regards those most beneficial and real principles relating to civil society and the reciprocal rights and duties of the sacred and the political powers. For, as there are on earth two principal societies, the one civil, the proximate end of which is the temporal and worldly good of the human race; the other religious, whose office it is to lead mankind to that true, heavenly, and everlasting happiness for which we are created; so these are twin powers, both subordinate to the eternal law of nature, and each working for its own ends in matters concerning its own order and domain. But when anything has to be settled which for different reasons and in a different way concerns both powers, necessity and public utility demand that an agreement shall be effected between them, without which an uncertain and unstable condition of things will be the result, totally inconsistent with the peace either of Church or State. When, therefore, a solemn public compact has been made between the sacred and the civil power, then it is as much the interest of the State as it is just that the compact should remain inviolate; because, as each power has services to render to the other, a certain and reciprocal advantage is enjoyed and conferred by each.
Source: Nobilissima Gallorum Gens (Vatican.va)