The Malachi Prophecies
According to the Old Testament, Elias the Prophet was raised "into heaven" in a "chariot of fire" (2 Kings 2:11). That made him the only prophet who never died. That also made him the only prophet who was "raised." This event supposedly happened about 850 years before the arrival of Jesus Christ. Later, in about 450 BC, the prophet Malachi prophesied that Elias would return to prepare the way for a risen Sun of Justice (Mal 3:20), meaning the Messiah, Jesus Christ.
According to Malachi's prophecy, Elias had to come before the Messiah. If Elias didn't come first, anyone who claimed to be the Messiah was of course a false prophet. The first Christians were very aware about the prophecy because all any Jew had to ask was ... Where is Elias? Since Elias had not returned at the time the gospel of Mark was written, Mark had to invent him. He did this through the character of John the Baptist. He relied on the population's belief in Angels (supernatural spirits), reincarnation (coming back from the dead in a new body), and transfiguration (the ability to change the appearance of a body). If you carefully read the entire Gospel of Mark (and Matthew), John the Baptist is first revealed as an Angel, then as a raised Prophet ... but like Jesus later says ... "he will not be recognised" (Mt 17:12).
The gospel of Mark viewed the following passages (shown in red) as "talking points" that needed to be fulfilled.
[1a] - The Angel of Malachi is the prophet Elias as revealed below in verse 23. For Mark and Matthew the Angel was John the Baptist who was supposed to be seen by the reader as a reincarnation of the prophet Elias.
[1b] - The "Lord whom you seek" is Jesus who "suddenly comes" to clease the temple in Mark 11:15-19 and Matthew 21:12-16.
 - The "day of his coming" is the day when Elias came to Jesus and transfigured him (Mk 9:1-8, and Mt 17:1-13). According to Matthew, when the apostles saw Jesus and Elias and heard God's voice from the cloud "they fell prostrate and vere very afraid" but then Jesus came and touched them and said "rise and do not be afraid." (Mt 17:6-7). This refers to when Jesus and Elias come together in the story of the transfiguration. The "face of Jesus shone like sun" (Mt 17:2) and his garments became "dazzling white such as no fuller on earth could bleach them" (Mk 9:3, and Mt 17:2).
 - This verse is the prophecy that God has a Son and that he will spare Men who serve (believe) in him.
 - The Sun of Righteousness is Jesus who will "rise" from the dead. The metaphor of "healing in it's wings" is a prophecy that Jesus would have the power to heal.
 - Since Malachi refers to Moses, the mountain, and then in the next verse to Elias, Mark had no choice but to incorporate Moses into his transfiguration story (Mk 9:4-5)
 - Elias the prophet (as John the Baptist) comes before Jesus on his great day, the day of his baptism. Elias also comes down to Jesus on his terrible day, the day of his crucifixion and death (to set him next to zero).Diagrams and Commentary for Mark 1:2-8