The mountain experience happened only once in a life time for Jesus' chosen apostles: Peter, James and John. Through the mountain experience, the apostles were certain that Jesus is God's Beloved Son, and through Him they came to learn about the immense bounty of God's love and mercy. Twice, they heard the same saying 'This is my Son, the Beloved, listen to him'. The first time was at Jesus' baptism (Mt 3,17), and the second time was at Jesus' Transfiguration (Mt 17,5). God identifies Jesus as God's only Beloved Son, and through him, we come to know God. The calling 'listen to him' made the apostles certain, that Jesus is the authentic voice of God's love and mercy. All other voices about God's love and mercy not in according with Jesus' teaching are unreal, and untrue. The apostles believed, those who listen to Jesus- 'listen to him', after their departure from this earth, would enjoy the life of glory like Moses and Elijah.
Second, the apostles had experienced, that God's brightness and greatness were too much for mortals to bear. Seeing the brightness of God coming through the cloud, the apostles all faced down, and trembled before God's glory. This seemed to suggest that a/ mortals are incapable of seeing the immortals, and b/ mortals are incapable of understanding the heavenly language. Jesus alone could, because he is the embodiment of God. However, when Jesus talked to his apostles and the crowds, he talked to them like a person talks to another person, and that through Jesus we come to have true knowledge of God. Listening to Jesus means listening to God.
Third, the apostles were certain, that there is life after death. Moses and Elijah, their Patriarchs, enjoyed new lives. The perishable nature needs to be transformed into the imperishable body. It is to be more the likeness of God, bright and shining. When a mortal nature changes into an immortal body that person has new life in Christ. Paul tells us, that this glorious new life is possible because the Risen Christ awarded those who 'listen to him' enjoyment of His triumph over the power of darkness. 1Cor 15,51-53).
Apart from these certainties, the apostles also experienced some uncertainties. First, they were unsure about: a, why Jesus instructed them to 'tell no one about the vision', and b, they were uncertain about what it meant 'until the Son Man has risen from the dead' Mt 17,9. Up in the mountain, God allowed the apostles to hear a portion of their conversation, the rest of the dialogue remained a mystery.
Second, Jesus is God's only Son, who had made many miracles to give life for God's people, and yet, he was the subject of being ridiculed by people who judged him, jeered by soldiers who whipped him, and of being challenged by a thief who was nailed on a cross beside him. Jesus, in his Passion, experienced the dreadful humiliation, suffering and a feeling of God's absence when he was hung on the cross. The apostles came to know, that glorification comes after, not before the suffering, but they lacked the understanding of the necessity of it. Both Jesus and the apostles enjoyed the experience up on the mountain, and both trembled when facing suffering. The fear of God's brightness and the darkness of brutality of man are the extreme poles of a human life.