Elder Paisios about Non-Chalcedonian and Heterodox

Anti-Chalcedonians:

Anti-Chalcedonians (that is, the Monophysites) — along with the other heretics and those of other religions — to be creatures of God and our brothers according to the flesh, in terms of our common descent from Adam; but he didn’t consider them childern of God and our brothers according to the Spirit, characterizations he applied only to Orthodox Christians. Regarding the Monophysite’s sympathizers and their fervent supporters among the Orthodox, he observed: 

“They don’t say that the Monophysites didn’t understand the Holy Fathers – they say that the Holy Fathers did not understand them. In other words, they talk as if they are right and and the fathers misunderstood them”. 
He considered proposals to erase from the liturgical books statements identifying Dioscorus and Severus as heretics to be a blasphemy against the holy fathers. 

He said:
“So many divinely enlightened holy fathers who were there at the time didn’t understand them, took them the wrong way, and now we come along after so many centuries to correct the holy fathers? And they don’t take the miracle of Saint Euphemia into account? Did she misunderstand the heretics’ tome too?" 

(Hieromonk Isaac: Elder Paisios of Mount Athos; 2012 For the English Language by the Holy Monastery of St. Arsenios the Cappadocian , pp. 659-660)

+++

Heterodox:

There’s no need for us to tell Christians who aren’t Orthodox that they’re going to hell or that they’re antichrists; but we also mustn’t tell them that they’ll be saved, because that’s giving them false assurance, and we’ll be judged for it. We have to give them a good kind of uneasiness — we have to tell them that they’re in error…In order for us to pray with someone, we must agree on the faith. 

(Elder Paisios of Mount Athos by Hieromonk Isaac pg. 658)

source