...no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good. Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural power and purpose sin against nature and commit a deed which is shameful and intrinsically vicious.
Casti Connubii
...the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it. To the one true Church of Christ, we say, which is visible to all, and which is to remain, according to the will of its Author, exactly the same as He instituted it. ... in this one Church of Christ no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors.
Mortalium Animos
If Socialism, like all errors, contains some truth (which, moreover, the Supreme Pontiffs have never denied), it is based nevertheless on a theory of human society peculiar to itself and irreconcilable with true Christianity. Religious socialism, Christian socialism, are contradictory terms; no one can be at the same time a good Catholic and a true Socialist.
Quadragesimo Anno
...if we are to avoid the errors which are the source and fountain-head of all the miseries of our time, the teaching of Aquinas must be adhered to more religiously than ever. ...against the much vaunted liberty of the human reason and its independence in regard to God he asserts the rights of primary Truth and the authority over us of the Supreme Master. It is therefore clear why Modernists are so amply justified in fearing no Doctor of the Church so much as Thomas Aquinas. Accordingly, just as it was said to the Egyptians of old in time of famine: "Go to Joseph," so that they should receive a supply of corn from him to nourish their bodies, so We now say to all such as are desirous of the truth: "Go to Thomas," and ask him to give you from his ample store the food of substantial doctrine wherewith to nourish your souls unto eternal life.
Studiorum Ducem
Whoever exalts race, or the people, or the State, or a particular form of State, or the depositories of power, or any other fundamental value of the human community - however necessary and honorable be their function in worldly things - whoever raises these notions above their standard value and divinizes them to an idolatrous level, distorts and perverts an order of the world planned and created by God; he is far from the true faith in God and from the concept of life which that faith upholds.
Mit brennender Sorge
...the Church, being a society of men, cannot exist or develop if it does not enjoy liberty of action, and that its members have the right to find in civil society the possibility of living according to the dictates of their consciences. ... Thus a Catholic will take care not to pass over his right to vote when the good of the Church or of the country requires it.
Nos es muy conocida
It would be a grave error, on the other hand, to say that Christ has no authority whatever in civil affairs, since, by virtue of the absolute empire over all creatures committed to him by the Father, all things are in his power. ... If to Christ our Lord is given all power in heaven and on earth; if all men, purchased by his precious blood, are by a new right subjected to his dominion; if this power embraces all men, it must be clear that not one of our faculties is exempt from his empire. He must reign in our minds, which should assent with perfect submission and firm belief to revealed truths and to the doctrines of Christ. He must reign in our wills, which should obey the laws and precepts of God. He must reign in our hearts, which should spurn natural desires and love God above all things, and cleave to him alone. He must reign in our bodies and in our members, which should serve as instruments for the interior sanctification of our souls ... .
Quas Primas
Communism is intrinsically wrong, and no one who would save Christian civilization may collaborate with it in any undertaking whatsoever. Those who permit themselves to be deceived into lending their aid towards the triumph of Communism in their own country, will be the first to fall victims of their error. And the greater the antiquity and grandeur of the Christian civilization in the regions where Communism successfully penetrates, so much more devastating will be the hatred displayed by the godless.
Divini Redemptoris
There exists an institution able to safeguard the sanctity of the law of nations. This institution is a part of every nation; at the same time it is above all nations. She enjoys, too, the highest authority, the fullness of the teaching power of the Apostles. Such an institution is the Church of Christ.
Ubi Arcano Dei Consilio
The Eucharistic Sacrifice in which the Immaculate Victim who taketh away the sins of the world is immolated, requires in a special way that the priest, by a holy and spotless life, should make himself as far as he can, less unworthy of God, to whom he daily offers that adorable Victim, the very Word of God incarnate for love of us. Agnoscite quod agitis, imitamini quod tractatis, "realize what you are doing, and imitate what you handle," says the Church through the Bishop to the deacons as they are about to be consecrated priests.
Ad Catholici Sacerdotii
What a spectacle for heaven and earth is not the Church in prayer! For centuries without interruption, from midnight to midnight, is repeated on earth the divine psalmody of the inspired canticles; there is no hour of the day that is not hallowed by its special liturgy; there is no stage of life that has not its part in the thanksgiving, praise, supplication and reparation in common use by the mystical body of Christ, which is the Church. ...
Nothing remains for Us, therefore, but to invite this poor world that has shed so much blood, has dug so many graves, has destroyed so many works, has deprived so many men of bread and labor, nothing else remains for us, We say, but to invite it in the loving words of the sacred Liturgy: Convertimini ad Dominum Deum vestrum. ["Be thou converted to the Lord thy God."]
Caritate Christi
