Eastern Theology has a word for the Holy Spirit’s work in salvation: deification (thēosis). This refers to the way that God in his grace restores the image of God in humans, making them share in the glorious and pure divine life, even as a distinction still remains between God and humans. There is a mystery here that we should come to terms with – we are the temple of God. God the Holy Spirit has taken up residence in our mortal bodies. Our bodies!
How awesome is this paradox? We are not God but God is in us and so we become holy and our very body, flesh and blood, becomes his dwelling place as we share in his life. We are renewed after his image. We are new creatures. A man named Henry Scougal wrote a book with a title that has always puzzled me: The Life of God in the Soul of Man. Wow! I get it now – God really lives within.
The reality of this should lead to humility and not pride, because the implications of this are both terrifying and encouraging. On the one hand, it is terrifying because of the high demand it places on us. For Paul, the reality that we are God’s temple means that we must be holy (1 Corinthians 6:18-20). Since God is in us, holiness is totally necessary! Holiness is essential! And we all know how hard it is to be holy.
On the other hand, since the Holy Spirit is in us, God is strengthening us by his glorious power (Colossians 1:11). The Holy Spirit is causing us to become holy and that is encouraging – we don’t live the Christian life alone. We have his presence. We have his weapons. We have his resources.
 Veli-Matti Kärkkäinen, Pneumatology (Grand Rapids, MI: Baker, 2002), Page 69-70.