The last promise
Malachi was the last of the prophets. His book closes with these words:
"Behold, I will send you Elias, the prophet, before the coming of the great and dreadful day of the Lord. And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers."
Malachias (Malachi) 4:5 to 6 · around 430 BC
Elias is the Douay-Rheims spelling of Elijah, the great prophet who never died. He was taken up to heaven in a whirlwind.
Then the prophets went silent. For about 400 years, no new prophet spoke in Israel.
What Elijah looked like
The Old Testament gives Elijah a description you can't mistake:
"But they said: A hairy man, with a girdle of leather about his loins. And he said: It is Elias, the Thesbite."
4 Kings (2 Kings) 1:8
A hair garment and a leather belt. That was enough for a king to recognise him on a description alone.
Who walked out of the desert
"And John himself had his garment of camels' hair, and a leathern girdle about his loins, and his food was locusts and wild honey."
Matthew 3:4
The silence broke with a man dressed exactly like Elijah, in the desert, calling Israel to turn back to God.
The angel said it first
Before John was even born, the angel Gabriel told his father what the boy would be. He quoted Malachi's own words:
"And he shall go before him in the spirit and power of Elias: that he may turn the hearts of the fathers to the children, and the incredulous to the wisdom of the just, to prepare for the Lord a perfect people."
Luke 1:17
Jesus said it plainly
"For all the prophets and the law prophesied until John: And if you will receive it, he is Elias that is to come."
Matthew 11:13 to 14
After the Transfiguration, his disciples asked about it again. He told them Elias had already come, "and they knew him not." Then "the disciples understood that he had spoken to them of John the Baptist" (Matthew 17:12 to 13).
The voice in the desert
Isaiah had seen him too, seven centuries early:
"The voice of one crying in the desert: Prepare ye the way of the Lord, make straight in the wilderness the paths of our God."
Isaias (Isaiah) 40:3
Notice who the way is being prepared for. Not a messenger. "The Lord." "Our God." John prepared the way, and Jesus walked down it.
Why this is a big deal
The prophets ended with one instruction: watch for Elijah. Four hundred years of silence later, a man dressed as Elijah came out of the desert, and right behind him came the one he was announcing.