Theosis (Greek: θέωσις), or deification (deification may also refer to apotheosis, lit. "making divine"), is a transformative process whose aim is likeness to or union with God, as taught by the Eastern Orthodox Church and the Byzantine Catholic Churches. As a process of transformation, theosis is brought about by the effects of catharsis (purification of mind and body) and theoria ('illumination' with the 'vision' of God). According to Eastern Christian teachings, theosis is very much the purpose of human life. It is considered achievable only through synergy (or cooperation) of human activity and God's uncreated energies (or operations).
According to Metropolitan Hierotheos (Vlachos), the primacy of theosis in Eastern Orthodox Christian theology is directly related to the fact that Byzantine theology (as historically conceived by its principal exponents) is based to a greater extent than Latin Catholictheology on the direct spiritual insights of the saints or mystics of the church rather than the often more seen as rational thought tradition of the West. Byzantine Christians consider that "no one who does not follow the path of union with God can be a theologian" in the proper sense. Thus theology in Byzantine Christianity is not treated primarily as an academic pursuit. Instead it is based on applied revelation (see gnosiology), and the primary validation of a theologian is understood to be a holy and ascetical life rather than intellectual training or academic credentials (see scholasticism).