What does 2 Corinthians 3:18 mean?

Paul has been referencing Exodus 34, which describes how the skin of Moses' face would shine after meeting with God. Moses' face so powerfully reflected God's glory that he covered it with a veil to protect the Israelites from even the reflection of God's presence with them. Their sin had caused their minds to be hardened. In that condition, God's glory was unbearable to them.

Paul has described the only way for this separation between sinful humans and God's glory to be removed. It is not through study or attempts at obedience or even through religiously following the law. God removes this "veil" between Himself and people only when they come to Him through faith in Christ. Only in Christ is sin forgiven once and for all. Only then does God's Holy Spirit act to remove the veil of hardness and allow those in Christ to see God's glory as He has revealed it in Jesus.

Now Paul shows that this moment of spiritual revelation is followed by spiritual transformation. With the veil gone, all those in Christ have unveiled faces, boldly looking at Christ, who is the glory of God. We can now see Him for who and what He is. This act of seeing, of understanding, the nature of Christ begins the process through which God transforms His children into the image of Christ.

Paul writes that this change happens from "one degree of glory to another" or "from glory to glory" in the Greek language. It is a remarkable promise. Not only are those in Christ finally freed to see God's glory, but they begin to become God's glory as they begin to become like Christ. This is not something we accomplish for ourselves through study or discipline or self-will. Paul insists that it is the Spirit of God who makes this happen in all of those who are in Christ Jesus. Seeing the glory of God in Christ initiates what some scholars call "progressive sanctification," meaning that we will become more and more like Him as we are changed by the Spirit over time.