Behold, I am sending My messenger to clear the way before Me, and the Lord whom you seek shall come to His Temple suddenly. As for the angel of the covenant that you desire, he is already coming. Malachi 3/1 Tanakh JPS 1985.
24He shall reconcile parents with children and children with their parents, so that, when I come, I do not strike the whole land with utter destruction.
Lo, I will send the prophet Elijah to you before
the coming of the awesome, fearful day of the Lord. Malachi 3/24 Tanakh JPS 1985.
31See, a time is coming—declares the Lord—when I will make a new covenant with the House of Israel and the House of Judah. 32It will not be like the covenant I made with their fathers, when I took them by the hand to lead them out of the land of Egypt, a covenant which they broke, though I espoused them—declares the Lord. 33But such is the covenant I will make with the House of Israel after these days—declares the Lord: I will put My Teaching into their inmost being and inscribe it upon their hearts. Then I will be their God, and they shall be My people. 34No longer will they need to teach one another and say to one another, “Heed the Lord“; for all of them, from the least of them to the greatest, shall heed Me—declares the Lord.
For I will forgive their iniquities,
And remember their sins no more. Jeremiah 31/31-34 Tanakh JPS 1985.
The main, though largely undiscussed, difficulty concerning the Day of the Lord is that of its significance. The passages do not convey a concept amenable to logical analysis, nor an eschatological doctrine. The warning is given that the Day of the Lord is near, but the more abstract idea involving history’s drawing to a close is not indicated. The wicked will be punished, justice established, mankind confounded, and its destiny somehow definitely changed. However, none of this seems essential to the notion itself. Nor is the concept related to expectations of theophany. The prophets simply confront their listeners with the awful certainty of future Divine action. Thus in the expression “Day of the Lord” there is a rather vague but stark and powerful concept: God will indeed act.
DAY OF THE LORD (edited) at jewishvirtuallibrary.org.
“And that through him [“My righteous servant”] the Lord’s purpose might prosper. Isaiah Chapter 53/10 Tanakh JPS 1985.
Elijah is to reconcile Jewish families to Judaism. If he does not G-d says He will smite the whole land with utter destruction. His purpose which might prosper is to turn the hearts of the fathers and the children back to Him.
G-d’s way of having “My Teaching” written on the hearts of men is through sin forgiveness and by sending a teacher with reasons to listen to the teacher. The teacher is the messenger of sin forgiveness of the Jewish people for a time to come that G-d declared in Jeremiah 31.
It is this sin forgiveness that causes His Teaching to be written on the hearts of men according to the covenant which ends with G-d saying “For I will forgive their iniquities, And remember their sins no more.”
This teacher is given knowledge of the new heaven G-d is creating for the Jewish people so that Israel’s name will endure forever of Isaiah 65. And he is described in Isaiah 53 and Isaiah 11/1-2. In Judaism he is known as HaMoshiach (G-d’s anointed king).
If He cannot do this with His prophet Elijah His purpose has failed. This is why the purpose of G-d through His righteous servant of Isaiah 53 “might” prosper. It is the same purpose as in Malachi 3. Reconcile the Jewish people to Judaism and HaShem or G-d will bring utter destruction to the land. G-d’s righteous servant of Isaiah 53 is Elijah.
When G-d says “I will put My Teaching into their inmost being and inscribe it upon their hearts” He has a man He can speak and write through to do it as G-d did with His servant Moses. Elijah is the prophet like Moses also. The people have to recognize the man and that is why there is a description of Elijah. The man G-d calls “My righteous servant” with whom G-d’s purpose might prosper in Isaiah 53.
And if His purpose does not prosper utter destruction will come to the whole land. G-d is to return to His Temple quickly in Jerusalem according to Malachi 3 which means the Temple has to be rebuilt. If there was a Temple as in the days of Jesus and John the Baptist (who Jesus claimed was Elijah) G-d would be there and not returning to it.
The repeating story of the destruction of the lands of Israel continues if the Jewish people once again refuse to recognize and listen to His prophet.
The prophets of the Tanakh confront their listeners with the awful certainty of a “Day of the Lord”. It is a vague but stark and powerful concept: God will indeed act. And He acts through men.
“Lo, I will send you Elijah the prophet before the coming of the great and awesome day of the Lord”.