We happen to be entrusted with a treasure — the theology of Orthodox Tradition. Orthodox theology is the culmination and product of centuries of experiences that have been repeated, renewed, and recorded by those who have experienced theosis at different times. We have the experience of the patriarchs and the prophets as well as the later experience of the Apostles. We call all of these experiences “glorification.” To say the prophet was glorified means that the prophet saw the glory of God. To say the Apostle was glorified means that the Apostle saw the glory of Christ. Seeing the glory of Christ, the Apostle ascertained by his own experience that the glory of Christ in the New Testament is the glory of God in the Old Testament. Hence, Christ is the Yahweh and the Elohim of the Old Testament.
Although it is not clear in the Old Testament Who the Holy Spirit is, the Apostles discovered Who He is by experience. Their experience repeats the experience of the prophets, but there is a difference because the Apostles were glorified after the Incarnation: Yahweh of the Old Testament now has the human nature of Christ. Although three of the Apostles were partially glorified during the Transfiguration on Mount Tabor, all of the Apostles were fully glorified at Pentecost, during which they reached the highest state of glorification that any human being can ever reach in this life.
After the experiences of the Apostles come the experiences of the glorified who include the Church Fathers and those saints who reached theosis. And so the experience of theosis continues to appear in each generation up to the present.  This experience of theosis is the core of the Orthodox tradition, the foundation of the local and ecumenical councils, and the basis for the Church’s canon law and liturgical life today.
If the contemporary Orthodox theologian is to acquire objectivity, he must rely on the experience of theosis. In other words, we can positively state that a student of Patristic tradition has acquired objectivity in his theological method only when he has personally undergone purification and illumination, and reached theosis. Only in this way will the researcher not only understand the Patristic tradition, but also verify for himself the truth of this tradition through the Holy Spirit.
63. During the past few decades many saints of the Church who have experienced theosis have become known, such as Elder Paisios the Hagiorite, Elder Sophrony of Essex, England, Elder Porphyrios of Athens, Elder Iakovos of Evia, Elder Joseph the Hesychast, and Elder Ephraim of Katounakia among others within and outside of Greece.
(Source): From Patristic Theology – The University Lectures of Father John Romanides (Thessaloniki, Greece: Uncut Mountain Press, 2008), pp. 92-94.)