The aim of the Christian life, which Seraphim described as the acquisition of the Holy Spirit of God, can equally well be defined in terms of deification. Basil described the human person as a creature who has received the order to become a god; and Athanasius, as we know, said that God became human that we humans might become god.
The saints, as Maximus the Confessor put it, are those who express the Holy Trinity in themselves. This idea of a personal and organic union between God and humans — God dwelling in us, and we in Him — is a constant theme in the Epistles of St Paul, who sees the Christian life above all else as a life ‘in Christ’. The same idea recurs in the famous text of 2 Peter: ‘Through these promises you may become partakers of the divine nature’ (i,4).
(Timothy Ware, The Orthodox Church, p. 225)