"Whom did you understand me as calling heretics? Those that deny the Son of God? Those that blaspheme the Holy Spirit and say that He is not God? Those that say the Father is greater than the Son? Those that confuse the Trinity into a monad or those who divide the God into three gods? Or, those who say Christ is the Son of God, but do not believe that He assumed flesh from a woman? Those who say the Father is without beginning and the Son as having a beginning and as being a creature...?
Perish the thought! I am not referring to any of these ungodly and godless persons that appeared as darkness but were overcome by the Holy Fathers who shined forth. The grace of the Holy Spirit shined through them and dissipated the darkness of those heresies, and to this day their inspired writings shine more brightly than the rays of the sun so that no one dares to speak against them.
Indeed those who observe the concordances of their Father and God are said to be deceived by demons. They blaspheme against the Holy Spirit that operates in them as at one time the Jews did to the Son of God. They saw demons being cast out by Christ by Christ and they blasphemed against His Holy Spirit, and those insolent persons shamelessly said, He cast out demons in the name of Beelzebul, prince of demons.
A heresy is to deviate from one of the established dogmas concerning our true faith, but to say that they who love God, are not sound, neither are the baptized sons of God and gods granted the privilege of receiving the Holy Spirit nor of possessing the Spirit in vision, in knowledge and in experience - such contenders who deny these things subvert the entire dispensation of God and of our Savior Jesus Christ... This is the worse heresy among all the heretics!
But I call heretics also those who say that in our own day and in our midst there is none who is able to observe the precepts of the Gospel and to become as were the holy Fathers. To begin with, the most reliable and practical consideration of all is the fact that faith is demonstrated by means of works in becoming illuminated and receiving the Holy Spirit and through the Holy Sprint beholding the Son together with the Father.
Those who contend that this is impossible are guilty of not simply one particular heresy, but of all heresies, and if one can say, exceeding and surpassing them all in ungodliness and excessiveness of blasphemy... Those who state such things, close heaven which Christ opened to us; and they obstruct the ascent to Him which He Himself initiated. Standing at the gates of heaven, bending down, seen of believers and declaring through the Gospel, He invites: Come unto me, all ye who labor and are heavy burdened and I will give you rest, while the antitheoi (God-opposing ones), or rather, the anti-christs, cry out, This is impossible! Impossible!" Saint Symeon The New Theologian