(Image retrieved from here)
There is a close relationship between Orthodoxy, the Church and the Divine Eucharist. Orthodoxy is the true faith of the Church and the Divine Eucharist is the true act of the Church. If there is a Church without Orthodoxy and the Eucharist, it is not a Church. If there is Orthodoxy outside the Church and the Divine Eucharist, it is not Orthodoxy. Moreover, if there is Divine Eucharist, without Orthodoxy and the Church, it is not the Divine Eucharist. This is why we maintain that outside the Orthodox Church there is no other Church, only heresies. Thus, the return of the heretics to the only true Church, the Orthodox Church, is needed.
-Metropolitan Hierotheos of Nafpaktos, Entering the Orthodox Church: The Catechism and Baptism of Adults, pp. 138-139.