This is one of the biggies in the gospels and so much has been said about it. I always found that cloud mysterious until I was reminded that the shechinah, the glory of God shows up prominently in Exodus. In a pillar leading the people, covering the mountaintop when Moses visits, etc.
And here is the cloud of God's presence again. Of course.
|Transfiguration, Alexander Andreyevich Ivanov|
Peter, James, and John did not see any ordinary cloud atop the mountain of transfiguration. They saw the cloud of God's Presence. In the Old Testament God's presence was made manifest to Israel in the form of a thick and luminous cloud. God guided the Israelites through the desert in a pillar of cloud (Exod 13:21-22). At Sinai the cloud of God's glory overshadowed the mountain when Moses received the Ten Commandments (Exod 24:15-18). Later the cloud filled the tabernacle (Exod 40:34). It also filled the temple in Jerusalem at its dedication by Solomon (1 Kings 8:11). However, in the sixth century BC, the prophet Ezekiel received a vision of God's glory cloud leaving the temple and Jerusalem because of the people's sinfulness (Ezek 10). Since that time, god's presence had not been visibly manifest to Israel But the prophets envisioned the return of this cloud to God's people in the eschatalogical age (2 Macc 2:8, Isa 4:5, Eze 43:1-5). Peter, James, and John witnessed the fulfillment of these prophetic hopes when the "bright cloud cast a shadow over them" on the mountain of transfiguration.Quote is from Catholic Commentary on Sacred Scripture: Gospel of Matthew by Curtis Mitch and Edward Sri. This series first ran in 2008. I'm refreshing it as I go.