"The all-important thing about this glorious feast is that it marks the point in time when three of the disciples, for a millioneth of a second, saw Jesus as he really was."
-- Canon Edward Nason West
As he really was. All the time. Remember how Moses had to veil his face after he'd been in the Presence? Because people couldn't bear the shining. How much more did Jesus have to "tone down" the glory, and only let it shine forth in flashes, fragments, glimpses here and there, when He walked the earth? He knew what his followers were ready for. He knew how much they could bear to see revealed. What a joy it must have been for Jesus, for just one brief moment on that earthly mountain, to hold nothing back, to shine forth in the fullness of his eternal glory! Even though it left the disciples dumbstruck, except for stammering Peter who gasped out the first words that came into his head...
And John. I think John must've remembered this moment, its incredible shining beautiful clarity, when he looked upon Jesus on the cross. I think perhaps John might have realized that he was seeing it again, the fullness of the glory, revealed this time not in light but in suffering.