Anyway, this excerpt, nos. 17-18 of the letter, is the entirety of section five, subtitled "Ecclesial Communion and Ecumenism". Pay close attention to what I've emphasized (with bold -- the italics are retained from the original):
17. "The Church knows that she is joined in many ways to the baptized who are honoured by the name of Christian, but who do not however profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter" (Lumen Gentium, 15). Among the non-Catholic Churches and Christian communities, there are indeed to be found many elements of the Church of Christ, which allow us, amid joy and hope, to acknowledge the existence of a certain communion, albeit imperfect (cf. Unitatis Redintegratio, 3a, 22; cf. Lumen Gentium 13d).True Catholic ecumenical dialogue is not content with recognizing our differences and moving on. True Catholic ecumenical dialogue isn't just dialogue! It's action that seeks to re-unite our separated brothers and sisters in Christ with the one, holy, catholic, and apostolic Church of Christ: the Roman Catholic Church.
This communion exists especially with the Eastern orthodox Churches, which, though separated from the See of Peter, remain united to the Catholic Church by means of very close bonds, such as the apostolic succession and a valid Eucharist, and therefore merit the title of particular Churches (cf. Unitatis Redintegratio, 14, 15c). Indeed, "through the celebration of the Eucharist of the Lord in each of these Churches, the Church of God is built up and grows in stature" (ibid., 15a), for in every valid celebration of the Eucharist the one, holy, catholic and apostolic Church becomes truly present (cf. supra, 5, 14).
Since, however, communion with the universal Church, represented by Peter's Successor, is not an external complement to the particular Church, but one of its internal constituents, the situation of those venerable Christian communities also means that their existence as particular Churches is wounded. The wound is even deeper in those ecclesial communities which have not retained the apostolic succession and a valid Eucharist. This in turn also injures the Catholic Church, called by the Lord to become for all "one flock" with "one shepherd" (John 10:16), in that it hinders the complete fulfilment of its universality in history.
18. This situation seriously calls for ecumenical commitment on the part of everyone, with a view to achieving full communion in the unity of the Church; that unity "which Christ bestowed on his Church from the beginning. This unity, we believe, 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" (Unitatis Redintegratio, 4c). In this ecumenical commitment, important priorities are prayer, penance, study, dialogue and collaboration, so that, through a new conversion to the Lord, all may be enabled to recognise the continuity of the Primacy of Peter in his successors, the Bishops of Rome, and to see the Petrine ministry fulfilled, in the manner intended by the Lord, as a worldwide apostolic service, which is present in all the Churches from within, and which, while preserving its substance as a divine institution, can find expression in various ways according to the different circumstances of time and place, as history has shown.