The nature of ortdhodoxy can be summed up in the sentence, much repeated by the Holy Fathers, That God became man so that man could become God. By taking the human nature, weaken by the Fall of Adam, the new Adam, Christ, could save man from the power of the devil and sin (death) to make her divine as she was meant to be from beginning.
All heresies without exception are in the ultimate analyzis the denial of this belief, and/or a denial on the way to achieve this end: Theosis, divinisation.
The heresy of arianism denied that Christ had the same nature as the Father, that he was created. How could this created Christ then be able to heal the sickness of sin? How could he make man divine since he himself is not of divine nature? The arianism taught that man transformed not to God but to the created nature of Christ.
The heresy appoloniarism teached that Christ did not take mans nous during the incarnation, but only had the nous of God. In this way man could not unite his nous to Gods nous and become divine.
Nestorianism taught that Gods Word coexisted with a great man Jesus. They denied the unification of Christ natures in the Word Of God and taught the unification of two humans, Gods human, and the Jesus human. This meant that the unification of Christ two natures wasnt true but, as the Fathers said, supreficial and delusive. This meant that man, we, cannot unite with God...the necessary condition to reach theosis.
The monophysits said that Christ nature, after the union with the divine nature, changed and lost his human properties...it disolved in the divine nature. Again, man could not unite with God, it was impossible because of the strong differences between the human and the divnie...and so no deification, no theosis.
The icon we venerate as orthodox is a confession of that God became man and could be painted as a man on icons. He, the true God that cannot be painted, become paintable, as the fathers expressed it. This is possible because he became true man and united man with Himself. Again, the iconoclasm attacked this because they taught that you cannot paint God or make images. In this is a deniel of the true man Jesus and so a deniel of the possibility to be man and God at the same time. The saint on icons is painted in their state of theosis...of divinisation. By attacking icons, the iconoclasts attacked this teaching.
Monothelitism denied that Christ had two wills a human and a divine.
The filioque heresy denied that the Holy Spirit has only the Father as sourse, and therby reducing the position of the Holy Spirit as a person. Latin theologians taught about the Holy Spriti like the mutual love between Father and Son. But it is only the Holy SPirit as a full person that unites us with Christ, and Christ who unites us with the Father and in this process make us divine. Later the pope claimed to be infallible, and only himself...it is a claim that the Holy Spirit only reveals absolute truth to One Bishop...and therefore makes the Fathers theology superflous...they did not have the Holy Spirit in the same way as the Pope had...The fact that they had reach tehosis, seemed to be of little importance
Roman Catholicism later denied the divine energies of God...and that man can become divine through prayer and hesychia...they focus on salvation as a purely juridical state...you are forgiven...but deny openly that man can become divine and see the uncreated Light on Tabor through prayer. This was the heresy of Barlaam, that later became the heresy of Scholasticism as such, and so the Heresy of the Latins till today.
And now, the false ecumenists deny the principles of the Church and therefore the Body of Christ...they deny Truth and compromise with it to achieve their man made interpretation of Christianity. They therefore deny the Church as we know it, the instrument (and not just this) for our divinisation...for theosis...saying that catholics and protestants are branches or that the Church does not yet exist....and thereby saying that all heresies that deny our divinisation are on a par with orthodoxy...
The nature of all heresies is the denyial of the principle of theosis, or an attack of the Chruch who is necessary for us to reach this state.
Therefore all heresies are from the Devil...the true enemy of our divinisation, the jeolus angel who could not accept our likeness of God and with every means try to distort the human nature by perversions to keep her in the fallen state for eternity...and thereby under his power.