I've been trying to write a post on Pope Leo XIII, but I finally decided the best thing is just to let him speak for himself by placing some quotes from one of his documents here. (The style of writing is a bit outdated, but the message is very up to date - if not prophetic.)
Excerpts from "Pope Leo XIII - Arcanum - On Christian Marriage - 10 February 1880"
"It is a reproach to some of the ancients that they showed themselves the enemies of marriage in many ways; but in our own age, much more pernicious is the sin of those who would fain pervert utterly the nature of marriage, perfect though it is, and complete in all its details and parts."
"...they attribute all power over marriage to civil rulers, and allow none whatever to the Church; and, when the Church exercises any such power, they think that she acts either by favor of the civil authority or to its injury. Now is the time, they say, for the heads of the State to vindicate their rights unflinchingly, and to do their best to settle all that relates to marriage according as to them seems good. "
"Nevertheless, the naturalists, as well as all who profess that they worship above all things the divinity of the State... cannot escape the charge of delusion. Marriage has God for its Author, and was from the very beginning a kind of foreshadowing of the Incarnation of His Son; and therefore there abides in it a something holy and religious; not extraneous, but innate; not derived from men, but implanted by nature."
"...marriage is holy by its own power, in its own nature, and of itself, it ought not to be regulated and administered by the will of civil rulers, but by the divine authority of the Church..."
"Let no one, then, be deceived by the distinction which some civil jurists have so strongly insisted upon—the distinction, namely, by virtue of which they sever the matrimonial contract from the sacrament.. A distinction, or rather severance, of this kind cannot be approved; for certain it is that in Christian marriage the contract is inseparable from the sacrament..."
"...it is clear that among Christians every true marriage is, in itself and by itself, a sacrament; and that nothing can be further from the truth than to say that the sacrament is a certain added ornament, or outward endowment, which can be separated and torn away from the contract at the caprice of man. Neither, therefore, by reasoning can it be shown, nor by any testimony of history be proved, that power over the marriages of Christians has ever lawfully been handed over to the rulers of the State. If, in this matter, the right of anyone else has ever been violated, no one can truly say that it has been violated by the Church. Would that the teaching of the naturalists, besides being full of falsehood and injustice, were not also the fertile source of much detriment and calamity! But it is easy to see at a glance the greatness of the evil which unhallowed marriages have brought, and ever will bring, on the whole of human society. "
"Now, those who deny that marriage is holy, and who relegate it, stripped of all holiness, among the class of common secular things, uproot thereby the foundations of nature, not only resisting the designs of Providence, but, so far as they can, destroying the order that God has ordained. No one, therefore, should wonder if from such insane and impious attempts there spring up a crop of evils pernicious in the highest degree both to the salvation of souls and to the safety of the commonwealth."
"...divorce once being tolerated, there will be no restraint powerful enough to keep it within the bounds marked out or pre-surmised. Great indeed is the force of example, and even greater still the might of passion. With such incitements it must needs follow that the eagerness for divorce, daily spreading by devious ways, will seize upon the minds of many like a virulent contagious disease, or like a flood of water bursting through every barrier."
"...all public society, will have unceasing cause to fear lest they should be miserably driven into that general confusion and overthrow of order which is even now the wicked aim of socialists and communists. Thus we see most clearly how foolish and senseless it is to expect any public good from divorce, when, on the contrary, it tends to the certain destruction of society."
[There are links to more of Pope Leo XIII's documents here. He lived from 1810 to 1903, and he was Pope from 1878 to 1903. He ushered the Church into the new century and prepared the Church for the an era in which the last monarchies would disappear from Europe.]