That is absolute rubbish. I am sorry to be so blunt (please forgive me

) but calling “deification”, the central dogmatic belief for orthodox Christian mysticism, “heresy” is unacceptable.
This is one of the most important patristic teachings. It was taught equally in the West as it was in the East. This doctrine is fundamental to the teaching of the Church Fathers, who held that “
God became man, so that man might become God”
St. Augustine, Sermo 13 de Tempore].
The entire purpose of the divine economy whereby the Son of God became incarnate was for the deification of man.
Being made in the image of God means that our soul is a spiritual “mirror” which, when cleansed through the fires of purification and wiped clean of the passions and sins which cloud it, reveals the Face of God to be within us as the very centre of our Being.
By grace, we pass beyond even this natural apprehension of God as our “centre” to find a life above ourselves where our reason cannot reach to the extent that we are led into the vey embrace of the love between the Three Persons and share in their Divine Life.
Such a union cannot be compared merely to two nations serving a common “purpose”.
While the Mormon error of thinking that human beings become mini-gods ruling over planets must be sternly avoided, divinization cannot be made into some “platitude” or purely symbolic idea like two separate individuals serving a common purpose. It is deep, ineffable communion of humanity with God in the risen Jesus. We are really incorporated into Christ’s body and made **partakers of the divine nature (2nd Peter 1:4). **
As Blessed Pope John Paul II explained:
This is the very essence of Christian mysticism, that we become by grace what God is by nature.
As Saint Catherine of Genoa explained in the Catholic tradition:
This in no way undermines the truth that God always remains personal as well as distinct from us in essence.
It is simply the realization of the fact that the purified soul, through the grace of God, is lifted up into the very life of the Trinity.
According to the Catechism of the Catholic Church:
The above quotation “102” is cited as being from a prayer by Blessed Elizabeth of the Trinity, a great Catholic mystic who died in 1906. Her very name signifies participation in the life of the Most Blessed Trinity. The Catholic approach to theosis is a distinctly Trinitarian experience, even when expressed at its most apophatic as with Meister Eckhart, and consists in sharing in the very unity of the Triune Persons through participation and grace, fulfilled in this life where we are called to become a house for the Most Blessed Trinity and more fully consummated after our deaths in the state of heaven when, in Latin understanding, we will have perfect sight of the Beatific Vision - the Divine Essence unknowable - and in the words of Ruusbroec, “
In this simple act of seeing we are therefore one life and one spirit with God.”
That is a good deal more than “serving a common purpose”.