Matthew 17:2

And he was transfigured before them; and his face did shine as the sun, and his garments became white as the light.

The glory of Christ was revealed. The effulgence of the Godhead made his face luminous and shone through his garments. Again from Williams,

It is a subjective vision that is here related, no mere inward impression on brain or nerve with nothing external to correspond, but a real objective occurrence, which was beheld by mortal eyes endued with no supernatural or abnormal powers, except insofar as they were enabled to look on this partial emanation of the divine effulgence.[5]

The heavenly glory of Christ irradiated his face and clothing, demonstrating his eternal nature in a way to make the apostles who witnessed it absolutely certain that Christ was God in human form. The profound impression made by the event was permanent. Long afterward, John wrote, "We beheld his glory, the glory as of the only begotten of the Father, full of grace and truth" (John 1:14).