Mountains, clouds, fear, and radiant transformation are familiar elements of biblical revelation. Moses and Elijah are especially associated with encounters with God at Sinai/Horeb.
Their presence creates a powerful scriptural context for understanding the event as more than an ordinary vision.
Exodus 33-34 describes Moses' encounter with God and the reflected radiance of his face; 1 Kings 19 describes Elijah's encounter at Horeb. Both narratives include divine 'passing by.'
The cloud recalls divine presence, while the Father's command to listen to Jesus echoes Deuteronomy 18:15 and the beloved-Son declaration recalls Psalm 2 and Isaiah 42.
Jesus' appearance is transformed and his garments become intensely radiant. Moses and Elijah appear with him, but the heavenly voice singles Jesus out: 'This is my beloved Son; listen to him.'
When the revelation ends, the disciples suddenly see no one with them except Jesus.
Radiance alone would not prove deity because created beings can reflect divine glory. The force comes from the entire theophanic configuration and Mark's broader narrative.
A recent scholarly treatment argues that the Transfiguration can be read as a revelation of Jesus' own divine glory, cohering with Mark's other YHWH-patterned scenes.
The disciples are frightened and do not know how to respond; the Father commands them to listen to Jesus.