The Augustinian understanding of the “efficacy” of the sacraments was never fully accepted in the Orthodox Church. Such an understanding of the sacraments is unacceptable for Orthodox tradition, for it is an understanding in which the grace within them is considered autonomous, independent of the Church. The sacraments can be performed only within the Church, and it is the Church that bestows efficacy, reality and salvation on them. In the Eastern Church, the attitude toward the sacraments of heretics and schismatics varied in different ages depending on the circumstances. The important role of evaluating this or that group that had separated itself from the Church provided a teaching opportunity: they approached those schisms that had caused the most damage to ecclesial unity.
That very rule acted in relation to heretics as to divergences from general church teachings on dogmatic issues. One of the most important Eastern Christian texts dedicated to this theme is the oration in honor of St. Athanasius of Alexandria, attributed to the hand of [St.] Gregory the Theologian. In this text, Gregory tells of the schism that was brewing between the Latin and Greek bishops on the question of triadological terminology and which Athanasius succeeded in preventing. In the course of the exposition, Gregory expresses a few precious ideas on the grounds of dogmatic disputes and schism between different Churches (the argument in this text revolves around the Council of Alexandria in 362, at which the question of the meaning of the triadological terms “hypostasis” and “essence” was raised:)
For as, in the case of one and the same quantity of water, there is separated from it, not only the residue which is left behind by the hand when drawing it, but also those drops, once contained in the hand, which trickle out through the fingers; so also there is a separation between us and, not only those who hold aloof in their impiety, but also those who are most pious, and both in regard to such doctrines as are of small consequence… and also in regard to expressions intended to bear the same meaning. We use in an orthodox sense the terms one Essence and three Hypostases, the one to denote the nature of the Godhead, the other the properties of the Three; the Italians mean the same, but owing to the scantiness of their vocabulary, and its poverty of terms, they are unable to distinguish between Essence and Hypostases, and therefore introduce the term Persons, to avoid being understood to assert three Essences. The result, were it not piteous, would be laughable. This slight difference of sound was taken to indicate a difference of faith. Then, Sabellianism was suspected in the doctrine of Three Persons, Arianism in that of Three Hypostases, both being the offspring of a contentious spirit. And then, from the gradual but constant growth of irritation (the unfailing result of contentionsness) there was a danger of the whole world being torn asunder in the strife about syllables… [Athanasius] conferred in his gentle and sympathetic way with both parties, and after he had carefully weighed the meaning of their expressions, and found that they had the same sense, and were in nowise different in doctrine, by permitting each party to use its own terms, he bound them together in unity of action. (Oration 21.35)
In the above text Gregory, first of all, stresses that the difference in dogmatic terminology does not always signify a discrepancy in the understanding of the same dogmata, and further not all dogmatic disputes arising between churches are the result of differences in faith; many of them were simply “a slightly different sound.” In other words, not every dogmatic discrepancy is absolutely a heresy. The history of the Church knows many instances when the confession of faith of one local church, translated into another language or concept in the context of a different theological tradition, is perceived as heretical and is rejected by another Church. Schisms arose on these grounds, eucharistic relations ceased between Churches, and their heads conferred anathemas on each other. Then time passed, and people understood that they were speaking in different tongues, yet professed one faith: then ecclesial relations were restored.
No less important is this other thesis: there exist insignificant (lit. “small”) dogmata, on the grounds for which discord is permissible. These are dogmata that, in Gregory’s opinion, can be “disregarded” for the sake of Church unity.
Now, for the third thesis contained in the text just quoted: it is frequently not only “those unfortunate ones” (heretics) who are severed from the Church, but also those “most pious Christians” who either rejected some kind of dogmatic formula suspected of containing heresy, or digressed into an incorrect understanding of one of these “small dogmata”. This thesis covers the essential difference between [St.] Gregory and [St.] Cyprian of Carthage, who considered only “wolves, dogs and snakes” to be separated from the Church. In Gregory’s mind, among those who have separated themselves from Church are those who remain faithful to it, although they turn out to be outside relationship with it. Not all Christians who separate themselves from the Church are one hundred percent heretics. The theologian demands tactfulness and vigilance in order to define whether or not there is this or that heretical teaching incompatible with general church teaching, or whether there is a discrepancy on the grounds of “small dogmata”, permissible within the unity of church tradition, or a “dispute on sounds” in general, arising as a result of misunderstanding and ignorance. (Orthodox Christianity: Doctrine and Teaching of the Orthodox Church, Vol. II. pp. 405-407)
see also: http://classicalchristianity.com/2013/12/20/on-the-reception-of-the-heterodox/
also here: http://classicalchristianity.com/2014/07/06/on-cyprianic-and-augustinian-theories-and-heterodox-sacraments/