In 1964 the Second Vatican Council published the Decree on Ecumenism, Unitatis Redintegratio, (Decree on Ecumenism). All texts on this page come from this document.
The Council recognizes the modern Ecumenical Movement, saying that,
Those belong to it who invoke the Triune God and confess Jesus as Lord and Savior, doing this not merely as individuals but also as corporate bodies. For almost everyone regards the body in which he has heard the Gospel as his Church and indeed, God's Church. All however, though in different ways, long for the one visible Church of God, a Church truly universal and set forth into the world that the world may be converted to the Gospel and so be saved, to the glory of God. (n.1)
Speaking of other Christians, the Council says,
For men who believe in Christ and have been truly baptized are in communion with the Catholic Church even though this communion is imperfect. The differences that exist in varying degrees between them and the Catholic Church-whether in doctrine and sometimes in discipline, or concerning the structure of the Church-do indeed create many obstacles, sometimes serious ones, to full ecclesiastical communion. The ecumenical movement is striving to overcome these obstacles. But even in spite of them it remains true that all who have been justified by faith in Baptism are members of Christ's body, and have a right to be called Christian, and so are correctly accepted as brothers by the children of the Catholic Church.(22)
Moreover, some and even very many of the significant elements and endowments which together go to build up and give life to the Church itself, can exist outside the visible boundaries of the Catholic Church: the written word of God; the life of grace; faith, hope and charity, with the other interior gifts of the Holy Spirit, and visible elements too. All of these, which come from Christ and lead back to Christ, belong by right to the one Church of Christ.
The brethren divided from us also use many liturgical actions of the Christian religion. These most certainly can truly engender a life of grace in ways that vary according to the condition of each Church or Community. These liturgical actions must be regarded as capable of giving access to the community of salvation.
It follows that the separated Churches(23) and Communities as such, though we believe them to be deficient in some respects, have been by no means deprived of significance and importance in the mystery of salvation. For the Spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Church. (n.3)
It states some principles for Ecumenical action,
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. (n.4)