Read Part 1, Part 2, Part 3.

"And, behold, there appeared unto them Moses and Elias talking with Him." Matthew 17:3
"And there appeared unto them Elias with Moses: and they were talking with Jesus." Mark 9:4
"And, behold, there talked with him two men, which were Moses and Elias:" Luke 9:30

Moses represents the Law and Elijah represents the Prophets. Mark's Gospel presents Elijah before Moses because his Gospel presents Christ as the Servant Prophet.

The Law commands but cannot save.
The Prophets reforms but cannot redeem.
Only Christ can both redeem and save.

The Law has its demands and the Prophets has its promises.
Christ is the fulfilment of the Law (Romans 10:4) and the Prophets (Acts 10:43).

Moses - foreshadow of the Law
Elijah - forecast of the Prophets
Christ Jesus - fullness of redemption