8 This change of heart and holiness of life, along with public and private prayer for the unity of Christians, should be regarded as the soul of the whole ecumenical movement, and merits the name, "spiritual ecumenism." It is a recognized custom for Catholics to have frequent recourse to that prayer for the unity of the Church which the Saviour Himself on the eve of His death so fervently appealed to His Father: "That they may all be one". In certain special circumstances, such as the prescribed prayers "for unity," and during ecumenical gatherings, it is allowable, indeed desirable that Catholics should join in prayer with their separated brethren. Such prayers in common are certainly an effective means of obtaining the grace of unity, and they are a true expression of the ties which still bind Catholics to their separated brethren. "For where two or three are gathered together in my name, there am I in the midst of them". Yet worship in common (communicatio in sacris) is not to be considered as a means to be used indiscriminately for the restoration of Christian unity. There are two main principles governing the practice of such common worship: first, the bearing witness to the unity of the Church, and second, the sharing in the means of grace. Witness to the unity of the Church very generally forbids common worship to Christians, but the grace to be had from it sometimes commends this practice. The course to be adopted, with due regard to all the circumstances of time, place, and persons, is to be decided by local episcopal authority, unless otherwise provided for by the Bishops' Conference according to its statutes, or by the Holy See.
Source: Unitatis Redintegratio (Vatican.va)