A most lucid and clear, and far-sighted, Instruction was issued by the Holy Office on Dec. 20, 1949.
It regarded ecumenism. It consists in a set of instructions and principles for authentic Catholic involvement in the ecumenical movement.
How greatly we need to re-receive this Instruction. Each and every Catholic should listen to the wisdom taught herein.
I paste a lengthy excerpt from EWTN’s translation, but also refer you to their site for more:
As regards <the manner and method of proceeding in this work>, the Bishops themselves will make regulations as to what is to be done and what is to be avoided, and shall see that these are observed by all. They shall also be on guard lest, on the false pretext that more attention should be paid to the points on which we agree than to those on which we differ, a dangerous indifferentism be encouraged, especially among persons whose training in theology is not deep and whose practice of their faith is not very strong. For care must be taken lest, in the so-called “irenic” spirit of today, through comparative study and the vain desire for a progressively closer mutual approach among the various professions of faith, Catholic doctrine-either in its; dogmas or in the truths which are connected with them-be so conformed or in a way adapted to the doctrines of dissident sects, that the purity of Catholic doctrine be impaired, or its genuine and certain meaning be obscured.
Also they must restrain that dangerous manner of speaking which generates false opinions and fallacious hopes incapable of realization; for example, to the effect that the teachings of the Encyclicals of the Roman Pontiffs on the return of dissidents to the Church, on the constitution of the Church, on the Mystical Body of Christ, should not be given too much importance seeing that they are not all matters of faith, or, what is worse, that in matters of dogma even the Catholic Church has not yet attained the fullness of Christ, but can still be perfected from outside. They shall take particular care and shall firmly insist that, in going over the history of the Reformation and the Reformers the defects of Catholics be not so exaggerated and the faults of the Reformers be so dissimulated, or that things which are rather accidental be not so emphasized, that what is most essential, namely the defection from the Catholic faith, be scarcely any longer seen or felt. Finally, they shall take precautions lest, through an excessive and false external activity, or through imprudence and an excited manner of proceeding, the end in view be rather harmed than served.
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.