Great insight from Robert Mounce, famed Greek teacher:
At the heart of the Christian experience is a radical transformation from what we were by nature into what God intends us to become by grace. Nowhere is that transformation stated with greater clarity than in II Cor 3:18. And at the heart of this verse is a present middle participle that reveals the secret of Christian growth and maturity.
What this verse tells us is that a wonderful change is taking place in the life of the believer. Although a veil remains over the mind of the unbeliever (v. 15), that veil is lifted for those who are in Christ (vv. 14, 16). They are being changed into the image of Christ from one degree of glory to the next.
The secret of divine transformation lies in the participle katoptrozomenoi. It comes from a verb which, in the middle, originally meant “to look into a mirror.” Then it came to mean “to gaze upon” or “to contemplate.” Taking the participle in the instrumental sense we read, “We all are being changed into the image of Christ by beholding the glory of the Lord.”
Transformation into the likeness of Christ is the inevitable result of gazing upon his glory. We become like that which dominates our thoughts and affections. Like Nathaniel Hawthorne’s “great stone face,” which shaped the life of the one who spent his days looking at that craggy representation of all that was held to be good and pure, so also does the believer gradually take on a family resemblance to his Lord as he spends his time contemplating the glory of God.
Note that the participle is present tense. It is a continual contemplation that effects the transformation. As the participle is present tense, so also is the finite verb “are being changed” (metamorfoumeqa). The transformation keeps pace with the contemplation. They are inextricably bound together. By continuing to behold the glory of the Lord we are continually being transformed into His image. (Mounce, William. Basics of Biblical Greek. Grand Rapids: Zondervan 2003, 245)
This last paragraph was most captivating in my mind. The looking and the changing are “bound together.” So the question lies, “am I changing?” From this verse, if my answer is “no” or “negligible” the reason why is “not enough looking.” You know you can read your Bible and not look? You can pray and not look? It all comes back to the heart. How many times did Christ condemn the Jews for heartless following? “These people honor me with their lips, but their hearts are far from me” – Matthew 15:8. Where are your heart and your mind when you are engaged in your devotions? Is it engaged in Christ? Or just on overdrive? Do you see the need in your life for “changing”? “Gaze”, Paul commands. One will not happen without the other.
And if this strikes conviction in anyone’s heart, I plead with you (as much as I plead with my own weak flesh), “do not be the double-minded man.” James warns earnestly of the consequences of looking into the perfect law of liberty, then going away and forgetting what kind of person you are (James 1:8, 22-25). Act now; make radically different choices, before it is too late.