It was fitting that the disciples should be afraid and fall down on hearing the voice of the Father, to show that the glory which was then being revealed surpasses in excellence the sense and faculty of all mortal beings; according to Exodus 33:20: "Man shall not see Me and live." This is what Jerome says on Matthew 17:6: "Such is human frailty that it cannot bear to gaze on such great glory." But men are healed of this frailty by Christ when He brings them into glory. And this is signified by what He says to them: "Arise, and fear not."
Aquinas ST 3.45.4 ad 4. Note that Exodus 33:20 cannot support what precedes it unless seeing the glory is looking on the divinity. Cf. 3.45.2 ("the clarity of Christ's body in His transfiguration was derived from His Godhead, as Damascene says, and from the glory of His soul" and "in Christ's transfiguration clarity overflowed from His Godhead and from His soul into His body"), 1.12.5 ("The disposition to the form of fire can be natural only to the subject of that form. Hence the light of glory cannot be natural to a creature unless the creature has a divine nature; which is impossible. But by this light the rational creature is made deiform"), and 2-1.5.6 ad 2 ("the light of glory, whereby God is seen, is in God perfectly and naturally; whereas in any creature, it is imperfectly and by similitude or participation").
