Today we commemorate the Baptism of Jesus.  WHY Jesus sought baptism at the hands of his kinsman John is intriguing, but the subject of another post.  Rather, I want to touch on the relationship between the sacrament of baptism and the process of theosis.  For this I turn to an excellent book by Norman Russell, Fellow Workers with God:  Orthodox Thinking on Theosis (Crestwood, St Vladimir's  Seminary Press 2009).  His point of departure is Psalm 82:  1; 6-7.  "God stands in the assembly of gods; in the midst of them he will judge gods...I said you are gods, and all of you sons of the Most High.  But you die like men, and fall as one of the princes."  This is one of the key scriptural texts referring to theosis.  On pp 61-63, he explores how the early fathers Irenaeus of Lyons and Clement of Alexandria interpreted this psalm.

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In his great work Against the Heresies, Irenaeus connects the gods of Psalm 82 for the first time with St Paul's teaching on adoption.  Psalm 82.1, he says...refers to the Father and the Son and those who have received the grace of adoption through which we cry "Abba, Father!"...Irenaeus says that Christians had become "gods" through baptism (Against the Heresies 3.6.1.)

A little later he moves on to consider verse 7 of the psalm.  This, he says, "was addressed to those who have not received the gift of adoption."  And by failing to honor the Incarnation through the acceptance of baptism, they have deprived themselves of their ascent to God (3.19.1).

When he returns to the psalm a third time, he develops an entirely new aspect.  Here he is looking for an argument against those who felt that having been baptized they had nothing more to do:  they had attained divinity in one go.  No, he said:  you have got to become fully human (ie, conquer the passions) before you can become like God.  You are not able to receive God's gift of eternal existence without first growing to maturity.  When the psalm says, "you shall die like men", that is to tell us that we cannot carry the full charge of divinity unless we first grow into the image and likeness that had been forfeited by Adam.  Baptism gives us a potential immortality, but we have to work at it before we can call ourselves "gods and sons of the Most High."  Irenaeus thus not only makes the psalm's connection with baptism explicit, but associates it with the recovery of the image and likeness of God (4.38.3).  In other words, to the psalm's connection with the mystery of baptism he adds a moral dimension...

...Clement [of Alexandria] ran a school, or study-circle...and published several books on the Christian life.  One of these was the Paedagogus, or Tutor, intended to help the recently baptized deepen their understanding of the Christian faith.  Here Clement follows Irenaeus in connecting Psalm 82.6 with baptism.  Christ, he says, at his own baptism in the Jordan was sanctified by the descent of the Holy Spirit:  "The same also takes place in our case, whose exemplar Christ became.  Being baptized, we are illuminated; illuminated, we become sons; being made sons, we become perfect; being made perfect we become immortal.  'I said,' says Scripture, 'you are gods and all of you sons of the Most High.' (Paed 1.26.1).

To become "gods and sons of the Most High" means to attain immortal life.  In his Protrepticus, however, or Exhortation to the Greeks, which is addressed to educated Christians and others interested in Christianity, Clement goes further.  Here he concludes by saying that the "gods and sons of the Most High" are not simply those whom the Father has adopted through baptism.  They are also those who have attained the likeness of God.  Irenaeus had already made this connection, but where Clement goes beyond him is  to link it with the Platonic axiom (drawn from Plato's dialogue, Thaeatetus 176b) that the philosopher's chief task is to become like God as far as possible.  Thus only the Christian is the true philosopher, because it is only through baptism in combination with the pursuit of the moral life that likeness to God can be attained.