15 Methodius especially did not hesitate to face misunderstandings, conflicts and even slanders and physical persecution, rather than fall short of his exemplary ecclesial fidelity, and in order to remain faithful to his duties as a Christian and a Bishop and to the obligations which he had assumed vis-a-vis the Church of Byzantium which had begotten him and sent him out as a missionary together with Cyril. Then there were his obligations to the Church of Rome, thanks to which he fulfilled his charge as Archbishop in "the territory of Saint Peter"; likewise his obligations to that Church growing in the lands of the Slavs, which he accepted as his own and successfully defended-convinced of his just-right before the ecclesiastical and civil authorities, protecting in particular the liturgy in the Old Slavonic language and the fundamental ecclesiastical rights proper to the Churches in the various nations. By thus acting, he always resorted, as did Constantine the Philosopher, to dialogue with those who opposed his ideas or his pastoral initiatives and who cast doubt on their legitimacy. Thus he would always remain a teacher for all those who, in whatever age, seek to eliminate discord by respecting the manifold fullness of the Church, which, conforming to the will of its Founder Jesus Christ, must be always one, holy, catholic and apostolic. This task was perfectly reflected in the Creed of the 150 Fathers of the Second Ecumenical Council of Constantinople, which is the unalterable profession of faith of all Christians.
Source: Slavorum Apostoli (Vatican.va)