St. Maximus the Confessor said: “Even if the whole universe holds communion with the [heretical] patriarch, I will not communicate with him. For I know from the writings of the holy Apostle Paul: the Holy Spirit declares that even the angels would be anathema if they should begin to preach another Gospel, introducing some new teaching.” The Life of St. Maximus the Confessor.
“Chrysostomos loudly declares not only heretics, but also those who have communion with them, to be enemies of God.” St. Theodore the Studite, Epistle of Abbot Theophilus.
“Guard yourselves from soul-destroying heresy, communion with which is alienation from Christ.” St. Theodore the Studite, P.G. 99.1216.
“Some have suffered final shipwreck with regard to the faith. Others, though they have not drowned in their thoughts, are nevertheless perishing through communion with heresy.” St. Theodore the Studite.