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.
Maybe an Eastern Catholic can elaborate, but my understanding of theosis is a unity with God (not becoming divine ourselves). For example, a nation might unite with other nations to serve a common purpose, without becoming part of those nations.
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:
“…Jesus is the new man (see Eph 4:24; Col 3:10) who calls redeemed humanity to share in His divine life. The mystery of the Incarnation lays the foundations for an anthropology which, reaching beyond its own limitations and contradictions, moves towards God Himself, indeed towards the goal of divinization. This occurs through the grafting of the redeemed on to Christ and their admission into the intimacy of the Trinitarian life. The Fathers have laid great stress on this soteriological dimension of the mystery of the Incarnation: it is only because the Son of God truly became man that man, in him and through him, can truly become a son of God…”
***- Blessed Pope John Paul II (Novo Millennio Ineunte, no. 23) ***
“…For the Son of God became man so that we might become God…For He became man that we might become divine…”
***- St. Athanasius (298 – 373), church father, De inc.54, 3: PG 25, 192B ***
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:
“…I will not be content until I am locked and enclosed within that divine heart in which all created forms lose themselves and, so lost, remain divine…My I is God, and I know of no other I than this my God…God became man in order to make me God; therefore I want to be changed completely into pure God…”
***- Saint Catherine of Genoa (1447-1510), Italian Catholic mystic ***
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:
260 The ultimate end of the whole divine economy is the entry of God’s creatures into the perfect unity of the Blessed Trinity.100 But even now we are called to be a dwelling for the Most Holy Trinity: “If a man loves me”, says the Lord, “he will keep my word, and my Father will love him, and we will come to him, and make our home with him”:101
O my God, Trinity whom I adore, help me forget myself entirely so to establish myself in you, unmovable and peaceful as if my soul were already in eternity. May nothing be able to trouble my peace or make me leave you, O my unchanging God, but may each minute bring me more deeply into your mystery! Grant my soul peace. Make it your heaven, your beloved dwelling and the place of your rest. May I never abandon you there, but may I be there, whole and entire, completely vigilant in my faith, entirely adoring, and wholly given over to your creative action.102
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”.