My point here will be to show that theosis is not taught by 2 Peter, as many have argued. The text reads.
I can pray this because his divine power has bestowed on us everything necessary for life and godliness through the rich knowledge of the one who called us by his own glory and excellence. Through these things he has bestowed on us his precious and most magnificent promises, so that by means of what was promised you may become partakers of the divine nature, after escaping the worldly corruption that is produced by evil desire. For this very reason, make every effort to add to your faith excellence, to excellence, knowledge; to knowledge, self-control; to self-control, perseverance; to perseverance, godliness; to godliness, brotherly affection; to brotherly affection, unselfish love. For if these things are really yours and are continually increasing, they will keep you from becoming ineffective and unproductive in your pursuit of knowing our Lord Jesus Christ more intimately. But concerning the one who lacks such things – he is blind. That is to say, he is nearsighted, since he has forgotten about the cleansing of his past sins. Therefore, brothers and sisters, make every effort to be sure of your calling and election. For by doing this you will never stumble into sin. For thus an entrance into the eternal kingdom of our Lord and Savior, Jesus Christ, will be richly provided for you. (2 Pet 1:3-11)
Verse 4 is the primary prooftext for theosis, but it clearly isn't teaching theosis. The point Peter is making here needs to be put in context.
First, the background context of 2 Peter, understood by what the book addresses itself, is an influence of apostates within the church who are arguing theology and ethics from the human perspective. Man is corrupt and incapable of living a godly life. That much is true. But the apostates use this idea in a man-centered view of holiness (as though holiness depends upon the human ability to achieve it) in order to argue that holiness isn't possible, so one should just accept his sinful behavior while in the body. In other words, these are the antinomians (likely of gnostic influence) who diminish the requirement for us to live a holy life by virtue of our corruption/inability to achieve it.
The textual references, then, are in relation to holiness and putting away sin. Verse 3 tells us that God's divine power has been given to us to achieve all life and godliness through the gospel promise that gets its power from His glory and excellence, not that of the human. Hence, being united to Christ unites us with God and His power and gives us the ability to live a life of faith, maturity, perseverance, godliness, self control, affection for the saints, sacrificial love, etc. And it is our being in Christ through the gospel call that gives us the Holy Spirit and endows us with the very power of God to overcome all sinful desire and pursue what is good in every aspect of life.
But this isn't theosis. This isn't a change of our ontology. This is our being joined to Christ and given the Spirit of God. In other words, this is discussing, not some spiritual evolution where our corrupt ontological nature becomes more divine. This is a matter of being in fellowship with the God from whom we were once alienated. We don't become more divine. We become more human, what humans were meant to be. The power never does come from us, then, but from God. It never is a shared nature with God's ontology, but a sharing of His power that has been given to overcome sin and live a life of holiness.
Theosis, therefore, ironically, may assume the very faulty emphasis upon man's inability, and thus his need to become a more divine man, that the heretics Peter addresses here assume. I'm not saying that theosis is heresy, of course (it can be); but this isn't a verse that argues for it either way.
In other words, we might look at God as the electricity that runs the appliance. The appliance never becomes the electricity. It merely uses it to power itself. The medium is the plug. Peter is arguing that we have been plugged into the divine electricity by virtue of the gospel outlet. Now, having been plugged in, we have all the power we need to power up a life of godliness and repentance from sin.
Indeed, we are changed by this, but again, this is a change that makes us more human, not more divine. Hence, we share in God's divine nature in terms of the power it provides us, not ontologically speaking. That's the context that is often ignored. The only evolution here is one where we are born again and mature from a sub-human creature to a fully-restored human as we were meant to be.
So to sum up: (1) the process is accomplished through the medium of the gospel and promises of the Word, not some direct experience with God in theoria; (2) there is nothing here that discusses our progression into something divine, or achieving some spiritual transcendence that goes beyond merely becoming the image of God, rather than "like God," as was the devil's lie in the beginning. (3) Instead, the passage is simply discussing that God has the ability to overcome sin and lead a holy life, and we, through the promises of the gospel, have access to Him and His power to do just that. By relying upon Him, rather than looking to become divine and achieve this ability for ourselves, this passage emphasizes our need for fellowship with God through the medium of the gospel/Scripture/Church provided (the rest of the book will discuss this as well).
EO's are, therefore, in error, and this error, once again, likely assumes the very heresies that were theoretically denounced by the very factions that now embrace them implicitly. But they will not do so with the aid of such passages as this.