It is critical, I believe, to once again reiterate Athanasius’ understanding that the redeemed person may become a son/daughter of God only by participation, which implies that far from being mechanical or automatic, the sonship of the redeemed is contingent and mutable: “From this it clearly appears that men can lose their sonship which they have by participation.
Although Athanasius is one of the key Fathers in the development of the Eastern spiritual idea of Theosis or deification, his thought is complex and, because of this fact, we need to step-back and take another look at this whole idea.
The Bible offers a sufficient number of passages about human participation in God for it to be taken as the important image of salvation. But perhaps it does not speak about it as much as the Eastern Fathers of the Church who presented cardinal texts for a foundation of Theosis. The two tests are 2 Peter 1:4 and Psalm 82:6 which Jesus cites in John 10:34-36a:

Through these he has given us his very great and precious promises, so that through them you may participate in the divine nature and escape the corruption in the world caused by evil desires (Peter)

I said, ‘You are gods; you are all sons of the Most High (Psalm)

Jesus answered them, “Is it not written in your Law, “I have said you are gods’?” If he called them gods, to whom the word of God came – and the Scripture cannot be broken – what about the one whom the Father set apart as he very own and sent into the world? (John)

The passage in 2 Peter accentuates one of the leading motifs in the Eastern Church’s understanding of salvation, namely release from the corruption and mortality caused by the evil desires of the world. Eastern theology does not focus so much on guilt as on mortality as the main problem of humanity. In addition, in the East, the concept of sin is viewed as something human beings do and choose for themselves rather than something “hereditary” as a result of the first human beings’ sin in the distant past. Cyril of   Alexandria comments on this passage from Peter to note that we are all called to participate in divinity, not just a few “saints”. Although Christ alone is God by nature, all people are called to become God by participation. In such participation we become likenesses of Christ and perfect images of God the Father.
The Eastern Church finds this doctrine of deification (Theosis) not only in the explicit texts just mentioned but elsewhere in the Bible, beginning with the Old Testament. To begin this quest, read Exodus 34:30 where Moses’ face is described as shining or Exodus 7:1 which reveals that Aaron became a god to Pharaoh. Also reread Matthew 17:4, another classic text.