The term “ecumenical movement” indicates the initiatives and activities planned and undertaken, according to the various needs of the Church and as opportunities offer, to promote Christian unity. These are: first, every effort to avoid expressions, judgments and actions which do not represent the condition of our separated brethren with truth and fairness and so make mutual relations with them more difficult; then, “dialogue” between competent experts from different Churches and Communities. At these meetings, which are organized in a religious spirit, each explains the teaching of his Communion in greater depth and brings out clearly its distinctive features. In such dialogue, everyone gains a truer knowledge and more just appreciation of the teaching and religious life of both Communions. In addition, the way is prepared for cooperation between them in the duties for the common good of humanity which are demanded by every Christian conscience; and, wherever this is allowed, there is prayer in common. Finally, all are led to examine their own faithfulness to Christ’s will for the Church and accordingly to undertake with vigor the task of renewal and reform.
When such actions are undertaken prudently and patiently by the Catholic faithful, with the attentive guidance of their bishops, they promote justice and truth, concord and collaboration, as well as the spirit of brotherly love and unity. This is the way that, when 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.
However, it is evident that, when individuals wish for full Catholic communion, their preparation and reconciliation is an undertaking which of its nature is distinct from ecumenical action. But there is no opposition between the two, since both proceed from the marvelous ways of God.
Catholics, in their ecumenical work, must assuredly be concerned for their separated brethren, praying for them, keeping them informed about the Church, making the first approaches toward them. But their primary duty is to make a careful and honest appraisal of whatever needs to be done or renewed in the Catholic household itself, in order that its life may bear witness more clearly and faithfully to the teachings and institutions which have come to it from Christ through the Apostles. – from Unitatis Redintegratio, decree of the Second Vatican Council (1964)