1) They deny the Holy Trinity
Protestant Trinity versus Orthodox Trinity.
Error 1) Jesus has his origin in the Father, and is thus "homoousios", but the Father has no origin.
The Orthodox have two false doctrines about the Holy Trinity. Few Orthodox realise many Protestants have a different view of the Trinity. The two choices are:
1) Protestant Trinity - That God the Son has always existed as God ("I am") as he is in the Trinity, and that Father and Son are simply the explanation of their relationship. Therefore Jesus has no origin whatsoever. He simply is. He is the great "I am that I am" just as much as the Father. He was not "born outside of time" or "born before the world" God strenuously avoids the word "born" as a definition of begotten. The word "firstborn" in the old testament was applied to Joseph after Ruben fell, and was applied to Jacob even though Esau was born physically first. The word "firstborn" therefore means "preeminent" it does not mean you must link the word "born" into the word "begotten" and then into Trinity in referring to the Son of God.
2) Orthodox Trinity - Like Catholics the Orthodox believe Jesus was quote "begotten of His Father before all worlds, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father … .”. When you probe their theology they say that God the Father has no origin, but Jesus has his origin in the Father, but before time and space. They say Jesus was "born before time" and thus this is not a creation.
The Orthodox like to accuse anyone not believing God the Son is "homoousios" as having the same doctrines as Arius. This very word "homoousios" is proof that they believe God the Son and the Holy Spirit have their origin outside of time in the Father. The entire word "substance" in regard to a purely spiritual being is entirely erroneous human based theology, not devine theology or bible based. They are saying Jesus is made out of God the Father, or drawn forth out of Him, but that these expressions are metaphors as this happened outside of time and space.
The error is deeply rooted in a wrong definition of "only begotten". If we call it a "heresy" not an "error" are we implying God can have no mercy on people who believe in the Trinity, but use incorrect concepts attached to the doctrine?
Error 2) Theosis.
The doctrine of Theosis denies the Holy Trinity, by adding multiple beings into God, and feminizes God by adding women into God by deification.
The Roman Catholics do not believe in this weird heresy, and no one presently (that I can discover) has isolated a date at which Orthodoxy decided to dive headlong into the doctrine, perhaps after the 1054 split, or when Catholicism - according to the Orthodox fairytale perspective - "fell away" from the teaching they were incredibly always supposed to have held.
My guess about the origin of Theosis - is that Orthodox bishops / theologists in their high minded process of spiritually dissecting God, and the (so called) hypostatic union, invented the idea that God is made up of ousia and energia (unsubstantiated) and felt that, wearing their crowns, uniforms, and what amounts to controlling the Holy Ghost by the heresy of epiklesis, vaingloriously perceived themselves as such spiritually advanced beings they should snatch the opportunity to deify themselves by a concept of becoming one with God, and Protestants do not accept their attempt to evade accusations of pride by saying this deification is via unity only in energia not ousia as the entire doctrine of that is heresy.
A further abomination to arise from the "ousia / energia" concept is that they also changed the word grace into "the uncreated energia of God" and then expanded the blasphemy into their priestcraft rituals (such as the Orthodox mass, chrismation etc) supposedly imparting the uncreated energia of God (pseudo-grace) into people via priests, bishops and their Titular Head of Orthodoxy, putting themselves once again in effect into beings in control of God Himself.
1) They deny the Holy Trinity - Theosis heresy 1. Theosis denies the Trinity, they have a different God. Theosis involves becoming united with God himself, not in a metaphor, but in reality, and their theological excuses that it is unity with God in energia (not in ousia) does not negate the blasphemy.
You will see later that I say Orthodox Theology is far more movable and indistinct in doctrine than Catholicism, and this topic is becoming such a total embarrassment to Orthodoxy they are starting to "manoeuvre" the concept of Theosis away from unity with God himself. Once we get them swapping and changing like that their whole house of cards will eventually fall over.
Theosis heresy 2 - the other evil doctrine regarding Theosis is dealt with later .Theosis is in effect making sanctification and spiritual growth into part of a process that contributes to actual salvation. (see later in the list of salvation errors)