It seems new to my spell check
How can we be deified, while divinity is only for God?
Basically, God shares His nature with us. Below I offer more detail and with scripture. But the simplicity is God's grace in us favors how Jesus is, by changing us to be and to love like Jesus. And Ephesians 5:2 confirms how Jesus is our example, required of us.
Now a longer explanation

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I just looked up how theosis is represented in a number of websites. The common meaning seems to be something like >
becoming in union with God . . . by being changed to be like God.
It is not only an Eastern Orthodox thing. Of course, how different people and groups represent it could be different. But there appear to be Episcopal people, too, who are into what they consider to be theosis.
I, of course, do not personally know these people so I can know if they believe in this as only a doctrinal teaching, or if they actually are being changed to become like God. Plus, if they consider themselves to be . . . theosized . . . how are they going about this? And is it really changing them to be more like Jesus?
About becoming in union with God > God's word says >
"But he who is joined to the Lord is one spirit with Him." (1 Corinthians 6:17)
So, being in union with God is a Bible basic > this is in God's word; so this is not some later thing which started with some group's church fathers or some group which named itself even over a century after Christ started the church.
I personally understand that if a person is
"one spirit with" Jesus, then how Jesus is will win out in the union, so the person becomes like Jesus. Because Jesus is almighty, and His grace is almighty to effect our nature so we become
"partakers of the divine nature", as 2 Peter 1:4 says > Peter says, here, that we have been given >
"exceedingly great and precious promises, that through these we may be partakers of the divine nature, having escaped the corruption that is in the world through lust." (in 2 Peter 1:2-4)
So, God's promises in His word are meant to bring us to share in God's own nature. This is in God's word. This is included in the basic meaning of God's promises, I would say we could rightly say, then.
Therefore, in my opinion, no distinct self-naming group is the only one into this. This is basic Bible, for all who have trusted in Jesus.
So . . . going with the Bible, I consider how Peter says a Christian has
"escaped the corruption that is in the world through lust." And now someone is sharing with Jesus in His nature. And how does God do this, please?
"Love has been perfected among us in this: that we may have boldness in the day of judgment; because as He is, so are we in this world." (1 John 4:17)
So, yes God's word says we in Jesus have become
"as He is" "in this world."
Not in some later life . . . but . . . now . . .
"in this world" we have been perfected in God's love, this scripture says. Our Apostle John, here, says
"we in this world" are
"as He is". John does not say, only certain hierarchical superiors or specially chosen saints, but
"we" have been perfected in God's own love; John our so great Apostle says
"we", and in this letter he says we are
"children of God" (1 John 3:1) and he addresses us as
"My little children" (1 John 2:1) and then brings us on to how we have been perfected in God's love so that
"we" are
"as He is" "in this world." So, John includes us children of God as
"we" with him > again > 1 John 4:17.
So, how does this work? What I get from the Bible is that we can see that a person escapes from
"the corruption that is in the world through lust," as 1 Peter 1:4 indicates. And this involves how our Father changes someone to be like His love, instead of how we have been while we could keep giving in to various lusts.
So . . . there is, I would say, being cured by God's love effecting our character. And yes every child of God has God's own love in our hearts >
"Now hope does not disappoint, because the love of God has been poured out in our hearts by the Holy Spirit who was given to us." (Romans 5:5)
So,
"in our hearts" . . . I understand . . . God's own love indeed is living and changing our real deep character to become like how God's own love is.
So, I would say > this is basic in God's word, but also in the actual personal experience of every child of God. You a child of God, then, can tell me from your actual experience what God's love is like, because of how this love has changed you to become like this . . . right?
For one thing > from the Bible and our experience, we know God is quiet . . . not silent. And in His love we have His
"gentle and quiet spirit, which is very precious in the sight of God." (in 1 Peter 3:4) So, in this love in the Holy Spirit, we experience God being quiet and gentle in us, in our emotions, in our feelings, in our thinking and talking and relating with one another >
"without complaining and disputing" > another Bible basic > in Philippians 2:13-16.
As you can see, we are going on and on with more and more scripture about this. I have only gotten started, with no quoting from anyone after our Canon Scripture writers. This is all Bible basic.
And in case you have trusted in Jesus, but you find yourself to be lacking in this > feed on Hebrews 12:4-14 > about how God our Heavenly Father does the correcting which brings this perfecting in His love. And keep trusting Him

Because He loves us, and therefore His grace favors us with such correction, in our character . . . not us merely struggling to make ourselves do things, but we are snuggling in His love.