The answer is: Moses and Elijah

Mark 9:4

The Transfiguration of Jesus is a deeply significant event recorded in Mark 9, and it showcases a powerful intersection of the Old and New Testaments. When Moses and Elijah appeared alongside Jesus, it wasn't just a dramatic moment; it symbolized the culmination of the Law and the Prophets. Moses, representing the Law, led the Israelites out of Egypt and received the Ten Commandments on Mount Sinai. Elijah, a renowned prophet, symbolizes the prophetic tradition of calling the people back to God and denouncing idolatry.

Interestingly, both figures had unique revelations; when God spoke to Moses, it was from a burning bush, and Elijah encountered Him through a "still small voice." Their presence at the Transfiguration emphasizes the importance of divine communication, which transcends immense historical and cultural gaps.

Moreover, consider this playful aspect: the idea of witnessing figures who never experienced death (Moses died, but his burial place remains unknown, and Elijah ascended into heaven in a whirlwind) captures the imagination. This makes the Transfiguration not just a miraculous encounter but an epic reunion of the ancient believers.

It's also worth noting that the disciples—Peter, James, and John—were understandably bewildered and overwhelmed, prompting Peter to suggest building shelters. His instinct was to honor a holy moment, yet Jesus knew it was about more than remaining there; it was about stepping down the mountain and sharing the message of love and redemption with the world.