How would you summarize the basic message of Christian salvation? If you look at the early theologians of the church from the second, third, fourth, and fifth centuries, you'll find a lot of statements of this sort: "For He was made man that we might be made God" (Athanasius, On the Incarnation 54). This is the doctrine of theosis or deification: God assumes human nature in order that humanity can become like God.
Of course, the ancient theologians did not imagine that we somehow merge with God to become a single being. There will always be a distinction between ourselves and God; but we will be raised from the lowly limitations and conditions in which we currently exist, and we will take on a mode of being that is more similar to God's. One of the ways in which we do this is by the resurrection: we no longer die after being resurrected, and in this way we participate in God's immortality.
Now I think the doctrine of theosis was so ubiquitous throughout the early centuries of the church because it is ubiquitous in the scriptures. One of the finest discussions of theosis comes in Paul's letter to the Ephesians:
For surely you have heard about him and were taught in him, as the truth is in Jesus. You were taught to put away your former way of life, your old self, corrupt and deluded by its lusts, and to be renewed in the spirit of your minds, and to clothe yourselves with the new self, created according to the likeness of God in true righteousness and holiness (Eph 4.21-24).
Here we find that becoming a Christian, as I've said before on other occasions, is a matter of becoming a new person. Now what exactly does this new person look like? Paul says that this new person is created in the likeness of God in true righteousness and holiness. This is clearly to say that our new identity is one that is like God's, with a mode of being and a way of life that is similar to God's. We become like small Gods in the world!
Of course, this is nothing new in the Bible; it wasn't invented by Paul, but it was taught in Genesis. There we read that mankind was created in the image and likeness of God (Gen 1.26-7). Mankind was created to be like God. The Greek word used in the Septuagint translation of Genesis is εἰκών, from which we get the word "icon." Human persons are meant to be living icons of God in the world, representations and presences of God in the creation.
This is what happens as we become Christians. It is a lifelong process, obviously, because we do not reach the pinnacle of righteousness immediately after baptism. But just as we should forget what lies behind us and what we were, we also must look forward to what we are becoming; and when we look to our goal, what else do we see except Christ, the image and likeness of the invisible God (Col 1.15)?