In a book called Reclaiming the Atonement, the author outlines the following reason, which sounds about right to me:
"Man was created to be joined with God in an intimate union, whereby he would be incorporated -- in the elevated measure divine grace makes possible to a human being -- into the very life of God. Man was created in order to be 'at-one' with God. Man was created for theosis. Theosis, then, is the true and proper ordo rerun [order of things]."
And "order of things" is correct, because it isn't just that man is ordered to God, but that the cosmos itself must be ordered to man if man is to be ordered to God. In a very real sense, man is the reason for the cosmos. This is why, for example, it is intelligible to us. But we can't just leave it at that; rather, we must ask why it is intelligible, and for what reason. Which goes back to the divine purpose.
As to the latter, Reardon further points out that we couldn't "share in the divine nature unless the Word shared a human nature," which is precisely why man's theosis requires God's Incarnation.
In the absence of the latter, we can still know that the cosmos is preternaturally ordered to us, but we couldn't know that we in turn are reciprocally ordered to its very creator.
Of the four causes -- material, efficient, formal, and final -- it is final cause that answers the question of why something exists. Eyes are for seeing; a car is for driving; hands are for grasping.
If you ask why cars exist, you can point to the materials of which it is composed, the people who built it, and its design, but none of these make sense without final cause: it was built in order to take us somewhere. Duh!
Note that the final cause is the last to be realized in time but the first to be contemplated in thought. I don't know how long it takes to realize a car from conception to fulfillment.
But it takes about 10 million years for solar-type stars to form, and about 10 billion for habitable planets. After that, life appears pretty quickly, but it takes another 3.5 billion years or so for self-conscious persons to arrive on the scene. It then took about 50 to 100,000 years to prepare man for the God-man.
And we've only had 2,000 years to assimilate him. As discussed in yesterday's post, the assimilation is ongrowing. As Kerouac said, walking on water wasn't built in a day.
So, when we come right down to it, everything exists for the sake of theosis. This makes perfect sense, because the purpose of something resides in its mature state. We all -- secular and religious alike - believe man "matures," the question being "how high?" -- does maturity extend all the way up, or end at some arbitrary point?
The latter makes no metaphysical sense, because a hierarchy is conditioned from the top down, i.e., it exhibits final causation.
Which helps to make sense of fallenness and sin, or at least looks at it from a slightly different angle.
By way of analogy, think of the concept of "pathology," which can only apply to living things. There cannot be a sick rock, although Al Franken comes close. The purpose of the heart, for example, is to pump blood. Anything that interferes with that -- clogged arteries, arrhythmias, valvular damage -- is pathological. Pathology only makes sense in light of final causation.
It has always been a pet peeve of mine that psychology attempts to speak of pathology in the absence of purpose. In reality it cannot be done. Rather, there will simply be an implicit and unarticulated purpose.
But ultimately, if theosis is man's purpose, then anything interfering with it will be pathological. On the spiritual plane, this is sin, precisely. Sin can only be understood in the context of what man is for. Sin, you might say, is "spiritual illness."
I'm thinking of a couple of Aphorisms:
The radical error — the deification of man — does not have its origin in history. Fallen man is the permanent possibility of committing the error.
And Radical sin relegates the sinner to a silent, gray universe, in which he drifts on the surface of the water, an inert castaway, toward inexorable insignificance.
Note that the deification of man goes to what was said above in paragraph three: it is to believe that the cosmos is mysteriously ordered to man, but to leave it at that -- to not realize that this is because man is ordered to God.
As to being relegated to that gray and silent universe, this is simply the logical consequence of denying God and hierarchy: it is as if the cosmos is ordered to man, but man is nothing. Therefore, everything is nothing.
Reardon notes that the early fathers were more aware of this than we seem to be. "The more traditional approach begins, not with fallen man, but with man in his Christian fulfillment: union with God."
So, our ultimate purpose explains sin better than sin explains the need for a sacrificial atonement.