see here), where all the encyclicals of Leo XIII are available—remarkably enough, considering that most of them contain doctrine to which later Magisterial pronouncements can be reconciled only with great difficulty.
Here are some marvelous excerpts to honor this anniversary.
Pope Leo XIII, Encyclical on the Duties of Christian Citizens, Sapientiae Christianae (1890)
6. [I]f we would judge aright, the supernatural love for the Church and the natural love of our own country proceed from the same eternal principle, since God Himself is their Author and originating Cause. Consequently, it follows that between the duties they respectively enjoin, neither can come into collision with the other. We can, certainly, and should love ourselves, bear ourselves kindly toward our fellow men, nourish affection for the State and the governing powers; but at the same time we can and must cherish toward the Church a feeling of filial piety, and love God with the deepest love of which we are capable. The order of precedence of these duties is, however, at times, either under stress of public calamities, or through the perverse will of men, inverted. For, instances occur where the State seems to require from men as subjects one thing, and religion, from men as Christians, quite another; and this in reality without any other ground, than that the rulers of the State either hold the sacred power of the Church of no account, or endeavor to subject it to their own will. Hence arises a conflict, and an occasion, through such conflict, of virtue being put to the proof. The two powers are confronted and urge their behests in a contrary sense; to obey both is wholly impossible. No man can serve two masters, for to please the one amounts to contemning the other.
7. As to which should be preferred, no one ought to balance for an instant. It is a high crime indeed to withdraw allegiance from God in order to please men, an act of consummate wickedness to break the laws of Jesus Christ, in order to yield obedience to earthly rulers, or, under pretext of keeping the civil law, to ignore the rights of the Church; “we ought to obey God rather than men.” This answer, which of old Peter and the other Apostles were accustomed to give the civil authorities who enjoined unrighteous things, we must, in like circumstances, give always and without hesitation. No better citizen is there, whether in time of peace or war, than the Christian who is mindful of his duty; but such a one should be ready to suffer all things, even death itself, rather than abandon the cause of God or of the Church.
10. But, if the laws of the State are manifestly at variance with the divine law, containing enactments hurtful to the Church, or conveying injunctions adverse to the duties imposed by religion, or if they violate in the person of the supreme Pontiff the authority of Jesus Christ, then, truly, to resist becomes a positive duty, to obey, a crime; a crime, moreover, combined with misdemeanor against the State itself, inasmuch as every offense leveled against religion is also a sin against the State. Here anew it becomes evident how unjust is the reproach of sedition; for the obedience due to rulers and legislators is not refused, but there is a deviation from their will in those precepts only which they have no power to enjoin. Commands that are issued adversely to the honor due to God, and hence are beyond the scope of justice, must be looked upon as anything rather than laws. …
14. But in this same matter, touching Christian faith, there are other duties whose exact and religious observance, necessary at all times in the interests of eternal salvation, become more especially so in these our days. … To recoil before an enemy, or to keep silence when from all sides such clamors are raised against truth, is the part of a man either devoid of character or who entertains doubt as to the truth of what he professes to believe. In both cases such mode of behaving is base and is insulting to God, and both are incompatible with the salvation of mankind. This kind of conduct is profitable only to the enemies of the faith, for nothing emboldens the wicked so greatly as the lack of courage on the part of the good. Moreover, want of vigor on the part of Christians is so much the more blameworthy, as not seldom little would be needed on their part to bring to naught false charges and refute erroneous opinions, and by always exerting themselves more strenuously they might reckon upon being successful. …
29. … Religion should, on the contrary, be accounted by everyone as holy and inviolate; nay, in the public order itself of States—which cannot be severed from the laws influencing morals and from religious duties—it is always urgent, and indeed the main preoccupation, to take thought how best to consult the interests of Catholicism. Wherever these appear by reason of the efforts of adversaries to be in danger, all differences of opinion among Catholics should forthwith cease, so that, like thoughts and counsels prevailing, they may hasten to the aid of religion, the general and supreme good, to which all else should be referred. We think it well to treat this matter somewhat more in detail.
30. The Church alike and the State, doubtless, both possess individual sovereignty; hence, in the carrying out of public affairs, neither obeys the other within the limits to which each is restricted by its constitution. It does not hence follow, however, that Church and State are in any manner severed, and still less antagonistic. Nature, in fact, has given us not only physical existence, but moral life likewise. Hence, from the tranquility of public order, which is the immediate purpose of civil society, man expects to derive his well-being, and still more the sheltering care necessary to his moral life, which consists exclusively in the knowledge and practice of virtue. He wishes, moreover, at the same time, as in duty bound, to find in the Church the aids necessary to his religious perfection, in the knowledge and practice of the true religion; of that religion which is the queen of virtues, because in binding these to God it completes them all and perfects them. Therefore, they who are engaged in framing constitutions and in enacting laws should bear in mind the moral and religious nature of man, and take care to help him, but in a right and orderly way, to gain perfection, neither enjoining nor forbidding anything save what is reasonably consistent with civil as well as with religious requirements. On this very account, the Church cannot stand by, indifferent as to the import and significance of laws enacted by the State; not insofar, indeed, as they refer to the State, but in so far as, passing beyond their due limits, they trench upon the rights of the Church.
31. From God has the duty been assigned to the Church not only to interpose resistance, if at any time the State rule should run counter to religion, but, further, to make a strong endeavor that the power of the Gospel may pervade the law and institutions of the nations. And inasmuch as the destiny of the State depends mainly on the disposition of those who are at the head of affairs, it follows that the Church cannot give countenance or favor to those whom she knows to be imbued with a spirit of hostility to her; who refuse openly to respect her rights; who make it their aim and purpose to tear asunder the alliance that should, by the very nature of things, connect the interests of religion with those of the State. On the contrary, she is (as she is bound to be) the upholder of those who are themselves imbued with the right way of thinking as to the relations between Church and State, and who strive to make them work in perfect accord for the common good. These precepts contain the abiding principle by which every Catholic should shape his conduct in regard to public life. …
32. Whence it appears how urgent is the duty to maintain perfect union of minds, especially at these our times, when the Christian name is assailed with designs so concerted and subtle. All who have it at heart to attach themselves earnestly to the Church, which is “the pillar and ground of the truth,” will easily steer clear of masters who are “lying and promising them liberty, when they themselves are slaves of corruption.” Nay, more, having made themselves sharers in the divine virtue which resides in the Church, they will triumph over the craft of their adversaries by wisdom, and over their violence by courage. This is not now the time and place to inquire whether and how far the inertness and internal dissensions of Catholics have contributed to the present condition of things; but it is certain at least that the perverse-minded would exhibit less boldness, and would not have brought about such an accumulation of ills, if the “faith that worketh by charity” had been generally more energetic and lively in the souls of men, and had there not been so universal a drifting away from the divinely established rule of morality throughout Christianity. May at least the lessons afforded by the memory of the past have the good result of leading to a wiser mode of acting in the future.
34. … [T]he enemies of the Church have for their object—and they hesitate not to proclaim it, and many among them boast of it—to destroy outright, if possible, the Catholic religion, which is alone the true religion. With such a purpose in hand they shrink from nothing, for they are fully conscious that the more faint-hearted those who withstand them become, the more easy will it be to work out their wicked will. Therefore, they who cherish the “prudence of the flesh” and who pretend to be unaware that every Christian ought to be a valiant soldier of Christ; they who would fain obtain the rewards owing to conquerors, while they are leading the lives of cowards, untouched in the fight, are so far from thwarting the onward march of the evil-disposed that, on the contrary, they even help it forward.
39. The Church, it is certain, at no time and in no particular is deserted by God; hence, there is no reason why she should be alarmed at the wickedness of men; but in the case of nations falling away from Christian virtue there is not a like ground of assurance, “for sin maketh nations miserable.” If every bygone age has experienced the force of this truth, wherefore should not our own? There are, in truth, very many signs which proclaim that just punishments are already menacing, and the condition of modern States tends to confirm this belief, since we perceive many of them in sad plight from intestine disorders, and not one entirely exempt. But, should those leagued together in wickedness hurry onward in the road they have boldly chosen, should they increase in influence and power in proportion as they make headway in their evil purposes and crafty schemes, there will be ground to fear lest the very foundations nature has laid for States to rest upon be utterly destroyed. Nor can such misgivings be removed by any mere human effort, especially as a vast number of men, having rejected the Christian faith, are on that account justly incurring the penalty of their pride, since blinded by their passions they search in vain for truth, laying hold on the false for the true, and thinking themselves wise when they call “evil good, and good evil,” and “put darkness in the place of light, and light in the place of darkness.” It is therefore necessary that God come to the rescue, and that, mindful of His mercy, He turn an eye of compassion on human society.