1) They deny the Holy Trinity
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)