The Orthodox view of sanctification (theosis) is not totally unique.  All Christian theology recognized  the difference between justification and sanctification. Deification (theosis) is both a transformative process as well as the goal of that process. The goal is the attainment of likeness to or union with God. As a process of transformation “theosis” is brought about by the effects of “catharsis” (purification of mind and body) and “theoria”

The Greek words for justification, the noun dikaiosune and verb dikaioo, refer to making something righteous. The Orthodox Study Bible defines justification as “the act whereby God forgives the sins of a believer and begins to transform him or her into a righteous person.”

What the righteousness of God is to justification the holiness of God is to sanctification. The Latin words from which the English words “sanctify” and “sanctification” are derived, namely, sanctus, “holy,” and facere, “to make,” help to make evident such a linguistic and conceptual relationship.

The Hebrew verb qadas orqades, “to be holy” or “to be clean,” is used chiefly in the OT. The Greek verb hagiazein, “to make holy” or “to sanctify,” is used more than two dozen times in the NT. The noun hagiasmos, “separation” or “sanctification,” is used five times (1 Thess 4:3, 4; 2 Thess 2:13; 1 Cor 1:30; 1 Pet 1:2), and the plural noun hagioi is used more than sixty times.

The Holy Scripture says, the primary meaning of sanctification is dedication or consecration. It involves a set-apartness that is basic to qadas and to hagios. The entire biblical conception of sanctification is rooted in the truth that God is “holy” and affirms that humans need to become “holy.” The set-apartness inherent in sanctification is intended to lead ultimately to Godlikeness. We understand that sanctification follows regeneration as growth follows birth; therefore, it is spiritual maturing following spiritual impartation.

Orthodox Christians have always understood that God calls His people not only to believe the truth He has revealed but to live according to that truth. When people place their trust in Jesus Christ, they are given a new nature and are indwelt by the Holy Spirit, who empowers them to live lives that honor God.

Christ reveals divine life through sanctification. Our  life is a gift of God that achieves its true purpose only in a close relationship with God Almighty. By having been sent into the world, Jesus Christ revealed the redemptive and sanctifying power of God. Jesus Christ invites the faithful to a fundamental repentance and spiritual renewal, to a constant martyrdom of love and witnessing by giving, as exemplified by all the saints.

SANCTIFICATION is being set apart for God. It involves us in the process of being cleansed and made holy by Christ in the Holy Spirit. We are called to be saints and to grow into the likeness of God. Having been given the gift of the Holy Spirit, we actively participate in sanctification. We cooperate with God, we work together with Him, that we may know Him, becoming by grace what He is by nature.

Faith in Christ sanctifies man through the energies of divine grace. The sanctification of each faithful person increases according to the degree of his or her faith. The more man is sanctified the more his mind is dedicated to eternal life, which he increasingly desires. Our union with Christ makes our life holy, providing a complete awareness that we are eternally bonded to him as beloved sons and daughters.

In casual terms, sanctification is our growth in relationship to Jesus Christ, a process that takes place after we have personally entered a relationship with Christ through faith. Justification brings us into the covenant of God, sanctification is our working out of that life (Phil 2:21) within this covenant relationship.

 Sanctification has one goal, that is the perfect love (1John4:7), Christ's love working through us. This can be done as we acquire the nature of Christ, and make his will as our own will. This perfection of love is a unity with Jesus Christ. This process is achieved by a "synergy" of God's work and our efforts. We cannot sanctification as our own, but God's working in us requires our free will and participation.

We Orthodox, then, ‘see ‘justification’ and ‘sanctification’ as one divine action…one continuous process’. “Justification is not a separate act of God but the negative aspect of salvation in Christ, which is freedom from sin, death and the devil; whereas sanctification is the positive act of God’s saving act, that of spiritual growth in new life in Christ communicated by God’s Holy Spirit.”

Justification is therefore Christ’s restoration to humanity - through His incarnation, death, and resurrection - of our potential for immortality and communion with God. Through Christ we no longer face eternal separation from God, but can be united to Him both now and for eternity (to learn more about this union, read the article on Salvation).