It is indeed a traditional teaching, taught by St Iraeneus, St Athanasius, St Gregory of Nyssa, St Thomas Aquina, St John of the Cross etc
From the RCCC art 1129
The Church affirms that for believers the sacraments of the New Covenant are necessary for salvation.51 “Sacramental grace” is the grace of the Holy Spirit, given by Christ and proper to each sacrament. the Spirit heals and transforms those who receive him by conforming them to the Son of God. the fruit of the sacramental life is that the Spirit of adoption makes the faithful partakers in the divine nature52 by uniting them in a living union with the only Son, the Savior.
From the Summa Theologica III q1 a2
*for Augustine says in a sermon (xiii de Temp.): "Go was made man, that man might be made God.
*
theosis = partaking in the divine nature.
But divinisation does not mean that our substance changes and literally fusionates with God, but that you become like “transparent” to God
Read this page of the Ascent of Mount by St John of the Cross where he explains what is theosis
ccel.org/ccel/john_cross/ascent.v.v.html
6. In order that both these things may be the better understood, let us make a comparison. A ray of sunlight is striking a window. If the window is in any way stained or misty, the sun’s ray will be unable to illumine it and transform it into its own light, totally, as it would if it were clean of all these things, and pure; but it will illumine it to a lesser degree, in proportion as it is less free from those mists and stains; and will do so to a greater degree, in proportion as it is cleaner from them, and this will not be because of the sun’s ray, but because of itself; so much so that, if it be wholly pure and clean, the ray of sunlight will transform it and illumine it in such wise that it will itself seem to be a ray and will give the same light as the ray. Although in reality the window has a nature distinct from that of the ray itself, however much it may resemble it, yet we may say that that window is a ray of the sun or is light by participation. And the soul is like this window, whereupon is ever beating (or, to express it better, wherein is ever dwelling) this Divine light of the Being of God according to nature, which we have described.
Also from The Apostolic Letter Oriental Lumen
vatican.va/holy_father/john_paul_ii/apost_letters/documents/hf_jp-ii_apl_02051995_orientale-lumen_en.html
- Certain features of the spiritual and theological tradition, common to the various Churches of the East mark their sensitivity to the forms taken by the transmission of the Gospel in Western lands. The Second Vatican Council summarized them as follows: “Everyone knows with what love the Eastern Christians celebrate the sacred liturgy, especially the Eucharistic mystery, source of the Church’s life and pledge of future glory. In this mystery the faithful, united with their bishops, have access to God the Father through the Son, the Word made flesh who suffered and was glorified, in the outpouring of the Holy Spirit. And so, made ‘sharers of the divine nature’ (2 Pt 1:4) they enter into communion with the most holy Trinity.”(11)
These features describe the Eastern outlook of the Christian. His or her goal is participation in the divine nature through communion with the mystery of the Holy Trinity. In this view the Father’s “monarchy” is outlined as well as the concept of salvation according to the divine plan, as it is presented by Eastern theology after Saint Irenaeus of Lyons and which spread among the Cappadocian Fathers.(12)
Participation in Trinitarian life takes place through the liturgy and in a special way through the Eucharist, the mystery of communion with the glorified body of Christ, the seed of immortality.(13) In divinization and particularly in the sacraments, Eastern theology attributes a very special role to the Holy Spirit: through the power of the Spirit who dwells in man deification already begins on earth; the creature is transfigured and God’s kingdom inaugurated.
The teaching of the Cappadocian Fathers on divinization passed into the tradition of all the Eastern Churches and is part of their common heritage. This can be summarized in the thought already expressed by Saint Irenaeus at the end of the second century: God passed into man so that man might pass over to God.(14) This theology of divinization remains one of the achievements particularly dear to Eastern Christian thought.(15)
On this path of divinization, those who have been made “most Christ - like” by grace and by commitment to the way of goodness go before us: the martyrs and the saints.(16) And the Virgin Mary occupies an altogether special place among them. From her the shoot of Jesse sprang (cf. Is 11:1 ). Her figure is not only the Mother who waits for us, but the Most Pure, who - the fulfillment of so many Old Testament prefigurations - is an icon of the Church, the symbol and anticipation of humanity transfigured by grace, the model and the unfailing hope for all those who direct their steps towards the heavenly Jerusalem.(17)*
Sounds Mormon or Hindu or New Age or something.