Thomas Aquinas

“ For the clarity of the soul overflows into a glorified body, by way of a permanent quality affecting the body. Hence bodily refulgence is not miraculous in a glorified body. But in Christ's transfiguration clarity overflowed from His Godhead and from His soul into His body, not as an immanent quality affecting His very body, but rather after the manner of a transient passion, as when the air is lit up by the sun. ”

“ The words quoted prove, not that the clarity of Christ was not that of glory, but that it was not the clarity of a glorified body, since Christ's body was not as yet immortal. And just as it was by dispensation that in Christ the glory of the soul should not overflow into the body so was it possible that by dispensation it might overflow as to the gift of clarity and not as to that of impassibility. ”

“ For everyone is a better witness of things that he knows. But at the time of Christ's transfiguration no one but the angels had as yet any knowledge from experience of the glory to come. Therefore the witnesses of the transfiguration should have been angels rather than men.
Objection 2: Further, truth, not fiction, is becoming in a witness of the truth. Now, Moses and Elias were there, not really, but only in appearance; for a gloss on Lk. 9:30, "They were Moses and Elias," says: "It must be observed that Moses and Elias were there neither in body nor in soul" ”