14 As we know, according to the teaching of the Second Vatican Council " the 'ecumenical movement' means those activities and enterprises which, according to various needs of the Church and as opportunities offer, are initiated and organized to promote Christian unity". Thus it seems in no way anachronistic to see Saints Cyril and Methodius as the authentic precursors of ecumenism, inasmuch as they wished to eliminate effectively or to reduce any divisions, real or only apparent, between the individual communities belonging to the same Church. For the division which unfortunately occurred in the course of the Church's history and which sadly still persists "not only openly contradicts the will of Christ, (but) provides a stumbling block to the world, and inflicts damage on the most holy cause of proclaiming the Gospel to every creature". The fervent solicitude shown by both Brothers and especially by Methodius by reason of his episcopal responsibility, to preserve unity of faith and love between the Churches of which they were members, namely, between the Church of Constantinople and the Church of Rome on the one hand, and the Churches which arose in the lands of the Slavs on the other, was and will always remain their great merit. This merit is all the greater if one takes into account the fact that their mission was exercised in the years 863-885, thus in the critical years when there emerged and began-to grow more serious the fatal discord and bitter controversy between the Churches of the East and the West. The division was accentuated by the question of where Bulgaria, which had just officially accepted Christianity, canonically belonged. In this stormy period, which was also marked by armed conflicts between neighboring Christian peoples, the holy Brothers from Salonika preserved a resolute and vigilant fidelity to right doctrine and to the tradition of the perfectly united Church, and in particular to the "divine teachings" and "ecclesiastical teachings" on which, in accordance with the Canons of the ancient Councils, her structure and organization was founded. This fidelity enabled them to complete their great missionary tasks and to remain in full spiritual and canonical unity with the Church of Rome, with the Church of Constantinople and with the new Churches which they had founded among the Slav peoples.
Source: Slavorum Apostoli (Vatican.va)