Luke chapter 09 (b)

34 While he was speaking, a cloud appeared and enveloped them, and they were afraid as they entered the cloud. 

When I read this paragraph, I was interested in the timing which the disciples got afraid. They were afraid as they entered the cloud, but not as they saw the vision of transformation. Reviewing the case of Moses:

29 When Moses came down from Mount Sinai with the two tablets of the Testimony in his hands, he was not aware that his face was radiant because he had spoken with the LORD.

30 When Aaron and all the Israelites saw Moses, his face was radiant, and they were afraid to come near him. 

Unlike Aaron and all the Israelites, the disciples' fear didn't came from facing the glory of Jesus. Instead, they can look at it. It's just the difference between the old testimony and the new one. In the age of new testimony, we can reflect God's glory with our unveiled faces whenever we turn to the Lord. That's the teaching in 2 Corinthians 3.13-18:

13 We are not like Moses, who would put a veil over his face to keep the Israelites from gazing at it while the radiance was fading away. 

14 But their minds were made dull, for to this day the same veil remains when the old covenant is read. It has not been removed, because only in Christ is it taken away. 

15 Even to this day when Moses is read, a veil covers their hearts. 

16 But whenever anyone turns to the Lord, the veil is taken away. 

17 Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. 

18 And we, who with unveiled faces all reflect the Lord's glory, are being transformed into his likeness with ever-increasing glory, which comes from the Lord, who is the Spirit. 

That's why the disciples can face Jesus's glory without fear, and even with joy. Not only because they have already turned to Lord, but also because they got progress in their faith. Let's consider the stages of these 3 disciple's faith. They have been chosen and have listened to Jesus's teachings(Luke6), they witnessed Jesus's power and miracles(Luke7.1-17; 8.22-56; 9-14-17) and they also had oral tests(Luke7.41-43; 9.13; 9.18-20) and internships(Luke8.23-25; 9.1-6). Therefore, they got progress day by day, and their intimacy with Jesus also developed. 

However, why do they still turn into fear later? 

That is the moment that they entered the cloud and cannot see Jesus for a while. That's the moment that they were afraid. Here comes a question: Why do God come in a figure of cloud? Why not in fire, in light, or just a voice or something else, but cloud? For me, I think the purpose might be to hide Jesus, or to seperate Jesus and the disciples, physically and mentally. Why do I think so?

Actually, since Peter identified Jesus as the Christ(Luke9.20), Jesus started to release the information of his death to disciples in many ways. This seperation held by cloud might just be an rehearsal of the day that Jesus truely leaves them away. Let's image the reactions of the disciples there:

"Oh God! Jesus disappered! What can we gonna do?" The disciples asked each other in a flurry. 

Just at that time!!!! :

35 A voice came from the cloud, saying, This is my Son, whom I have chosen; listen to him. 

So they pricked up their ears and listened carefually. And then.....

36 When the voice had spoken, they found that Jesus was alone.[...]

Finally, they got peaceful because whenever they calm down to LISTEN to Jesus's tender voice and Jesus's words, instead of always SEEing the reality, they could find that Jesus is still there alone. Nothing can seperate us from God's love, unless we first got lost in the cloud of worries or sins. In brief, LISTEN to Jesus, in our daily life, and also in our prayers, then we can naturally SEE his power, his glory and his comfort before us.