Such was the "rallying cry" of Ancient Christianity, according to Prof. John Peter Kenney (2018, p.107). This statement pithily (and provocatively) summarises the Christian doctrine of the Incarnation. In particular, it connects nature of the Incarnation ('God became man') with its aim ('that we might become God'). It might seem natural to begin by explicating the former, before moving on to the latter. But sometimes the best way to understand what a thing is to look at what it does. We might therefore benefit from starting with what the Incarnation is supposed to accomplish, and work our way backwards from there.
The result of the Incarnation, then, is that it enables human beings to 'become God'. This obviously shouldn't be taken to mean that we are to become our own 'Gods'. The mere fact that we are and always will be creatures necessarily precludes this, since the divine essence includes absolute independence, and thus uncreatedness. In any case, Scripture offers some clarification on this point: "he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature" (2 Pet. 1:4). We are called to 'become God' in the sense of having a share in his Being, i.e. in his perfect Goodness.
This isn't quite sufficient. Indeed, we saw in another article that God creates by sharing his Being with his creatures, which may be said to reflect or participate in the Being of God, each in their own way. If that's right, then it seems that all creatures partake of the divine nature, just by virtue of being creatures.
But even so, the extent of the partaking varies from creature to creature. Not all creatures reflect God's being to the same extent. For example, since God is perfectly rational, creatures with rational capacities share in more of his Being than creatures that don't.
So, being called to 'become God' must mean being invited to share in far more of God's Being than that which we already partake in as rational creatures -- specifically, to participate in his divine life. To see as he sees, to love as he loves, to experience his perfect joy and peace, and to do so eternally.
The incarnation, then, is the means by which God offers the gift of Himself to humanity. This is true not only in the sense of sharing his divine life with us, but also in the sense of allowing us to enter into perfect fellowship with God, the Supreme Good, whereby we 'know Him fully, just as we are fully known', to paraphrase St Paul (1 Cor 13:12). This is why the ultimate outcome of the incarnation is spoken of as a marriage between redeemed humanity and the incarnate God (Rev 19:7-9). In fact, the two senses go hand in hand: we cannot possibly enjoy intimate union with God without being sufficiently like him, any more than mice can have deep friendships with human beings. Conversely, we cannot be changed into his likeness without allowing him to pour his life into us, which itself presupposes some degree of union.
Origen of Alexandria compares this process to a piece of iron acquiring the characteristics of fire when place in it:
"If, then, a mass of iron be kept constantly in the fire, receiving the heat through all its pores and veins, and the fire being continuous and the iron never removed from it, it become wholly converted into the latter (...) In the same way, that soul which, like an iron in the fire, has been perpetually placed (...) in God, is God in all that it does, feels, and understands"
This brings us to my final question: how exactly is it that God's incarnation in Christ brings about these things? Though I hope to write more about this in future articles, I still have some space for a few vague comments about this. The event of the Incarnation, as understood by historic Christianity, involved the conjoining of a divine nature -- the eternal Word of the Father -- to a particular human nature. This enabled said human nature to achieve what humans had thus far failed to do, i.e. to live a life of pure self-giving love and willing obedience to God, even to the point of death on a cross. This opened a way to reconciliation and union with God, so that all human beings could embrace the perfect intimacy that Christ enjoys with his Father. In Christ's own words,
"All that the Father has is mine; therefore I said that he will take what is mine and declare it to you." John 16:15
In other words, the Incarnation does not strictly end with Christ -- rather, it continues in the life of his people, who receive the divine life that he himself receives from his Father. Thus we have come full circle: God was made man, so that we might be made God.