I know Whom I have believed, and I am certain that He is able to keep that which I have committed to Him, against that day; being a just judge. (Psalm) Lord, Thou hast proved me and known me: Thou hast known my sitting down, and my rising up.
Today closes the Octave of Prayer for Christian Unity, begun on the Feast of the Chair of St. Peter in Rome. Yesterday, I offered a (surely incomplete) list of magisterial documents on the subject of ecumenism, but figuring that if one who is a full time student of theology doesn't have time to read them all, others also might not be able to, I'll offer here a few of my personal favorite quotes (not intended to be fully representative of the contents).
~From Pope Pius IX, Encyclical Letter to the Bishops of Italy Quanto conficiamur moerore (On Promotion of False Doctrines), 10 August 1863.
God forbid that the children of the Catholic Church should even in any way be unfriendly to those who are not at all united to us by the same bonds of faith and love. On the contrary, let them be eager always to attend to their needs with all the kind services of Christian charity, whether they are poor or sick or suffering any other kind of visitation. First of all, let them rescue them from the darkness of the errors into which they have unhappily fallen and strive to guide them back to Catholic truth and to their most loving Mother who is ever holding out her maternal arms to receive them lovingly back into her fold.
~From Pope Pius XI, Encyclical Letter Mortalium animos (On Religious Unity), 6 January 1928.
So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for 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. During the lapse of centuries, the mystical Spouse of Christ has never been contaminated, nor can she ever in the future be contaminated…
Let, therefore, the separated children draw nigh to the Apostolic See, set up in the City which Peter and Paul, the Princes of the Apostles, consecrated by their blood; to that See, We repeat, which is "the root and womb whence the Church of God springs," not with the intention and the hope that "the Church of the living God, the pillar and ground of the truth" will cast aside the integrity of the faith and tolerate their errors, but, on the contrary, that they themselves submit to its teaching and government. Would that it were Our happy lot to do that which so many of Our predecessors could not, to embrace with fatherly affection those children, whose unhappy separation from Us We now bewail. Would that God our Savior, "Who will have all men to be saved and to come to the knowledge of the truth," would hear us when We humbly beg that He would deign to recall all who stray to the unity of the Church!
~From The Holy Office, Instruction On the Ecumenical Movement, 20 December 1949.
Therefore the [whole] and [entire] Catholic doctrine is to be presented and explained: by no means is it permitted to pass over in silence or to veil in ambiguous terms the Catholic truth regarding the nature and way of justification, the constitution of the Church, the primacy of jurisdiction of the Roman Pontiff, and the only true union by the return of the dissidents to the one true Church of Christ. It should be made clear to them that, in returning to the Church, they will lose nothing of that good which by the grace of God has hitherto been implanted in them, but that it will rather be supplemented and completed by their return. However, one should not speak of this in such a way that they will imagine that in returning to the Church they are bringing to it something substantial which it has hitherto lacked. It will be necessary to say these things clearly and openly, first because it is the truth that they themselves are seeking, and moreover because outside the truth no true union can ever be attained.
~From the Council of Vatican II, Session V, Decree Unitatis redintegratio (On Ecumenism), 21 November 1964.
[W]hen the obstacles to perfect ecclesiastical communion have been gradually overcome, all Christians will at last, in a common celebration of the Eucharist, be gathered into the one and only Church in that unity which Christ bestowed on His Church from the beginning. We believe that this unity subsists in the Catholic Church as something she can never lose, and we hope that it will continue to increase until the end of time. (4)
It is, of course, essential that the doctrine should be clearly presented in its entirety. Nothing is so foreign to the spirit of ecumenism as a false irenicism, in which the purity of Catholic doctrine suffers loss and its genuine and certain meaning is clouded. (11)
~From Pope John Paul II, Encyclical Letter Ut unum sint (On Commitment to Ecumenism), 25 May 1995.
The unity willed by God can be attained only by the adherence of all to the content of revealed faith in its entirety. In matters of faith, compromise is in contradiction with God who is Truth. In the Body of Christ, "the way, and the truth, and the life" (Jn 14:6), who could consider legitimate a reconciliation brought about at the expense of the truth? (18)
Love for the truth is the deepest dimension of any authentic quest for full communion between Christians. (36)
Full communion of course will have to come about through the acceptance of the whole truth into which the Holy Spirit guides Christ's disciples. Hence all forms of reductionism or facile "agreement" must be absolutely avoided. Serious questions must be resolved, for if not, they will reappear at another time, either in the same terms or in a different guise. (36)
~From the Congregation for the Doctrine of the Faith, Doctrinal Note On Some Aspects of Evangelization, 3 December 2007.
There is today, however, a growing confusion which leads many to leave the missionary command of the Lord unheard and ineffective (cf. Mt 28:19). Often it is maintained that any attempt to convince others on religious matters is a limitation of their freedom. From this perspective, it would only be legitimate to present one's own ideas and to invite people to act according to their consciences, without aiming at their conversion to Christ and to the Catholic faith. It is enough, so they say, to help people to become more human or more faithful to their own religion; it is enough to build communities which strive for justice, freedom, peace and solidarity. Furthermore, some maintain that Christ should not be proclaimed to those who do not know him, nor should joining the Church be promoted, since it would also be possible to be saved without explicit knowledge of Christ and without formal incorporation in the Church. (3)
Everywhere and always, each Catholic has the right and the duty to give the witness and the full proclamation of his faith. With non-Catholic Christians, Catholics must enter into a respectful dialogue of charity and truth, a dialogue which is not only an exchange of ideas, but also of gifts, in order that the fullness of the means of salvation can be offered to one's partners in dialogue. In this way, they are led to an ever deeper conversion to Christ. (12)