By Protopresbyter Fr. George Metallinos
Heresy is not just an error of the reasoning faculty, nor are heretics simply misplaced in finding the truth. In their case something happens deeper and more substantial. They know the Bible to the letter, often in amazing ways. But they lack something essential, and this lack radically distinguishes them from the Fathers. They lack the experience of the Holy Spirit of the Fathers, the interior illumination of the Spirit. This is because they have not gone through the therapeutic process of the Church.
Externally they may be morally flawless. But they do not have within them the Spirit. Therefore, they do not see what the Fathers see in the Spirit. Though intellectually they may be amazingly developed.
Indeed it is a fact, that all the great heretics impressed with their vast knowledge and "wisdom". Even today! But they don't have a pure heart, nor have they been changed into temples of the Holy Spirit.
Heresy presupposes poor to non-existent therapeutic treatment. For this reason the theology of heretics is intellectual - a scientific hypothesis, logical, and a syllogistic game. The experience of theosis, which adorns the Fathers, is what they are missing. This is why heretics cannot distinguish at critical points truth from deceit. For they cannot see the truth within them, they do not know it in their heart. Without the vehicle of "noetic prayer" they cannot arrive at "glorification", which is the revelation of "all truth" from the Holy Spirit.
Herein is also revealed the tragedy of all heretics, especially the heresiarchs. Unillumined themselves, they seek to illumine. Unhealed themselves, they seek to heal. Atheists themselves (being without the true God), they seek to theologize. We could liken heretics to quacks and charlatans, who scam. But they are worse: they are physicians who offer fatal therapeutic treatment, who kill people for eternity. They are pharmacists who market poisoned drugs, corrupted, that are hazardous to public health, not physically, but spiritually and eternally.
Source: Translated by John Sanidopoulos.