125 Governments can assist the Church greatly in the execution of its important office, if, in laying down their ordinances, they take account of what is prescribed by divine and ecclesiastical law, and if penalties are fixed for offenders. For as it is, there are those who think that whatever is permitted by the laws of the State, or at least is not punished by them, is allowed also in the moral order, and, because they neither fear God nor see any reason to fear the laws of man, they act even against their conscience, thus often bringing ruin upon themselves and upon many others. There will be no peril to or lessening of the rights and integrity of the State from its association with the Church. Such suspicion and fear is empty and groundless, as Leo XIII has already so clearly set forth: "It is generally agreed," he says, "that the Founder of the Church, Jesus Christ, wished the spiritual power to be distinct from the civil, and each to be free and unhampered in doing its own work, not forgetting, however, that it is expedient to both, and in the interest of everybody, that there be a harmonious relationship. . . If the civil power combines in a friendly manner with the spiritual power of the Church, it necessarily follows that both parties will greatly benefit. The dignity of the State will be enhanced, and with religion as its guide, there will never be a rule that is not just; while for the Church there will be at hand a safeguard and defense which will operate to the public good of the faithful."
Source: Casti Connubii (Vatican.va)