A brief excerpt from this chapter in the Glory book…
Taken together, 2 Corinthians 3:18 and Romans 12:1-2 cover the waterfront when it comes to transformation, for they tell us that such change involves the work of the Spirit (2 Cor 3) as well as our own commitment (Rom 12). A contradiction? Not really, but rather two sides of the same coin, or perhaps better, Paul’s way of explaining that transformation calls for a partnership: the work God wants to do proceeds as those in whom He is active agree and comply.
The third place this verb appears is in descriptions of Jesus’ transfiguration (Matthew 17:2; Mark 9:2). That might be a coincidence, but my suspicion is that Paul knew of this usage in talking about what happened with Jesus on that mountain, and that he consciously chose the verb when wanting to discuss the transformation of Jesus’ disciples.
While it’s precarious to speak on behalf of Paul—to say with certainty: “here’s what Paul meant”—I’ll hazard a guess as to why he chose this particular way of talking about what happens when the Spirit takes up residence in a person. The English word, with its prefix of trans-, suggests a change (think: translate, transfer, transcontinental), but of a particular kind. It is not as though we start with a watermelon and wind up with a waterpark, but rather, that a transformation occurs when a thing starts with one shape and ends up having another. Modeling clay might offer a good illustration: the lump of soft material that can be fashioned into a horse or a tree or a coffee mug—but that remains the same lump of clay, no matter what the potter does.
In Jesus’ case, ‘transfiguration’ describes a change in the typical appearance of Jesus, but not a change in His essence. That is, when He shines, it is not as though what accounts for the shining—namely, Jesus’ glory—has somehow been missing up to this point such that it must be laid upon or into Him during that stint on the mountain. To say this in a different way, Jesus’ glory—or more accurately, certain manifestations of His glory—might have been hidden, but it was not absent.
If this is a reasonable rendering of the verb, and a reasonable interpretation of what was going on at Jesus’ transfiguration, then I would go on to suggest that Paul’s use of the verb to speak of human ‘transformation’ is to highlight the idea that people, because of the Spirit’s activity and their own complicity with the Spirit, are being changed—but not from one thing to something else wholly different. Rather, the Spirit is restoring what should have been the case all along. That is, humans created by God were made in His image and thus with His glory; with the ‘fall’ came radical shifts (people now “fell short” of glory, to recall Romans 3:23) but not irrevocable damage. With the pouring out of God’s grace such that the Spirit might reside in those rescued from sin, humans could be restored, infused once more with the fullness of glory.
The rest of this chapter (draft) is here, at the bottom of the page.