The gospel reports of the transfiguration tell us that the disciples saw something very different from what they had been able to see before. I've always understood that to mean that the appearance of Jesus changed. As Mark says, "He was transfigured...and his clothes became dazzling white."
But, what about us? We've heard about Jesus. We know about the crucifixion and we know about the resurrection. Is transfiguration interesting only as an historical event that happened to some other people at some other time in some other place?
Paul was writing to the early church, the post-transfiguration, post-crucifixion, post-resurrection church: Those other apostles have been misleading you. They have thrown a veil over the true gospel. The light that has enabled us to see is the light that will enable you to see. That light comes in the glory of Christ, who is the image of God.
The light was there for the Corinthians, but they had allowed themselves to be blinded by the false apostles.