9 We must get to know the outlook of our separated brethren. To achieve this purpose, study is of necessity required, and this must be pursued with a sense of realism and good will. Catholics, who already have a proper grounding, need to acquire a more adequate understanding of the respective doctrines of our separated brethren, their history, their spiritual and liturgical life, their religious psychology and general background. Most valuable for this purpose are meetings of the two sides - especially for discussion of theological problems - where each can deal with the other on an equal footing - provided that those who take part in them are truly competent and have the approval of the bishops. From such dialogue will emerge still more clearly what the situation of the Catholic Church really is. In this way too the outlook of our separated brethren will be better understood, and our own belief more aptly explained.
Source: Unitatis Redintegratio (Vatican.va)