3 It is not difficult to determine what would be the form and character of the State were it governed according to the principles of Christian philosophy. Man's natural instinct moves him to live in civil society, for he cannot, if dwelling apart, provide himself with the necessary requirements of life, nor procure the means of developing his mental and moral faculties. Hence, it is divinely ordained that he should lead his life-be it family, or civil-with his fellow men, amongst whom alone his several wants can be adequately supplied. But, as no society can hold together unless some one be over all, directing all to strive earnestly for the common good, every body politic must have a ruling authority, and this authority, no less than society itself, has its source in nature, and has, consequently, God for its Author. Hence, it follows that all public power must proceed from God. For God alone is the true and supreme Lord of the world. Everything, without exception, must be subject to Him, and must serve him, so that whosoever holds the right to govern holds it from one sole and single source, namely, God, the sovereign Ruler of all. "There is no power but from God."(1)
Source: Immortale Dei (Vatican.va)