St. Augustine most wisely says: “I think that you can see, at the same time, that there is nothing just and lawful in that temporal law, unless what men have gathered from this eternal law.” If, then, by anyone in authority, something be sanctioned out of conformity with the principles of right reason, and consequently hurtful to the commonwealth, such an enactment can have no binding force of law, as being no rule of justice, but certain to lead men away from that good which is the very end of civil society. Therefore, the nature of human liberty, however it be considered, whether in individuals or in society, whether in those who command or in those who obey, supposes the necessity of obedience to some supreme and eternal law, which is no other than the authority of God, commanding good and forbidding evil. And, so far from this most just authority of God over men diminishing, or even destroying their liberty, it protects and perfects it, for the real perfection of all creatures is found in the prosecution and attainment of their respective ends; but the supreme end to which human liberty must aspire is God.

These precepts of the truest and highest teaching, made known to us by the light of reason itself, the Church, instructed by the example and doctrine of her divine Author, has ever propagated and asserted; for she has ever made them the measure of her office and of her teaching to the Christian nations. As to morals, the laws of the Gospel not only immeasurably surpass the wisdom of the heathen, but are an invitation and an introduction to a state of holiness unknown to the ancients; and, bringing man nearer to God, they make him at once the possessor of a more perfect liberty. Thus, the powerful influence of the Church has ever been manifested in the custody and protection of the civil and political liberty of the people. The enumeration of its merits in this respect does not belong to our present purpose. It is sufficient to recall the fact that slavery, that old reproach of the heathen nations, was mainly abolished by the beneficent efforts of the Church. The impartiality of law and the true brotherhood of man were first asserted by Jesus Christ; and His apostles re-echoed His voice when they declared that in future there was to be neither Jew, nor Gentile, nor barbarian, nor Scythian, but all were brothers in Christ. So powerful, so conspicuous, in this respect is the influence of the Church that experience abundantly testifies how savage customs are no longer possible in any land where she has once set her foot; but that gentleness speedily takes the place of cruelty, and the light of truth quickly dispels the darkness of barbarism. Nor has the Church been less lavish in the benefits she has conferred on civilised nations in every age, either by resisting the tyranny of the wicked, or by protecting the innocent and helpless from injury, or, finally, by using her influence in the support of any form of government which commended itself to the citizens at home, because of its justice, or was feared by their enemies without, because of its power.

–  Leo XIII, Libertas