Those who knowingly commune with heresy are equated with those who preach it.

In contrast to the Arch-Heresiarch Bartholomew and all his … underlings, that is, those who commemorate him and therefore commune with him, the Arch-Heresiarch of the Last Times, the Atlas of Orthodoxy [St. Mark of Ephesus] thunderously proclaims concerning the Frankish Latins: 

"We have rejected them as heretics, and for this reason, we have separated from them," "They are heretics, therefore we have cut them off as heretics."

But the same, the steadfast Saint, thunderously proclaims also against the Graeco-Latins, that is, those who accept the Frankish Latins as "Orthodox," namely the Graeco-Latin Phanariots and those who commemorate the Graeco-Latin Patriarch Bartholomew and thus commune with him: 

"But what," he says, "shall we do with these intermediaries, the Graeco-Latins? ... Flee from them as one flees from a serpent, as from those very ones... the Christ-merchants and Christ-traders. For these are, according to the divine Apostle, those who regard piety as gain, concerning whom he adds, saying: 'Withdraw from such people.... What communion has light with darkness? Or what portion has a believer with an unbeliever?...'" "Therefore, flee from them, brethren, and from communion with them. For such as these (i.e., the Latin-minded bishops) are false apostles, deceitful workers, transforming themselves into apostles of Christ..." (Patrologia Orientalis, Volume XV, Au Concile de Florence, pp. 318-320). 

The same is confirmed by Saint Basil the Great:

"Those who pretend to confess the sound faith but commune with those of another mind, if they do not separate after being admonished, should not only be considered as excommunicated but also not even be called brethren." (P.G. 160, p. 101). 

The Fathers teach that those who knowingly commune with heresy are equated with those who preach it. Can you tell us to whom the Holy Fathers address all these words? To the rocks, the walls, and the pebbles? No, of course not, but to you and me, to those registered in the records of the Church. And what do they mean by "communion"? They mean participation, joint prayer, co-worship, any sacramental and prayerful relationship with them, even commemoration. Even the commemoration of heretics defiles the faithful. These ones, they should not only be considered as excommunicated (walled off) but not even called brethren. FLEE, THEREFORE, FROM THEM AND FROM COMMUNION WITH THEM. The Holy Fathers use the same words. Are we the ones misinterpreting them? You give your own interpretation.

"So that this may not happen, be willing... to turn away from those who openly hold impious beliefs and to guard yourselves against those who claim not to hold the beliefs of Arius yet commune with the impious. And especially from those whose mindset we reject, it is fitting to flee from their communion... And if someone pretends to confess the right faith but is seen communing with them, exhort such a one to abstain from such a habit; and if he promises to do so, consider him as a brother. But if he stubbornly persists, reject such a one. For by continuing in this manner, you will keep the faith pure; and they, seeing you, will benefit, fearing lest they be regarded as impious and holding the same beliefs as them." (St. Athanasius, "Letter to Those Practicing the Monastic Life," P.G. 26, 1185-1188) 

St. John Chrysostom writes: 

"Never think highly of yourself, even if you are wiser than all; nor despise the counsel of others, even if they are the most insignificant of all. If someone advises you about something proper, even if he is a servant, accept the admonition. For we should not look at the quality of the persons giving counsel, but rather always at the nature of the counsel itself. If someone advises you correctly, even if he is a servant, accept the admonition; but if he advises something destructive, even if he holds the highest rank, reject his opinion. For it is not the quality of the persons advising that we must consider, but rather the very nature of the counsel itself everywhere." 

Cheap piety does not lead to the Kingdom of God.

"I know many who fast, pray, endure hardships, and generally practice every religious custom with zeal—provided that it costs them nothing (cheap piety)—and yet they do not give a single coin to those in need. What benefit does the rest of their virtue bring them? They are not fit for the Kingdom of God." (St. Basil the Great, P.G. 31, p. 277, "To the Rich")

Greek source: https://apotixisi.blogspot.com/p/blog-page.html