Mal 4:5-6 "Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD. And he will turn the hearts of the fathers to the children, and the hearts of the children to their fathers, lest I come and strike the earth with a curse."
The book of Malachi revealed the depravity of the Jews just before the great and dreadful day of the Lord. They robbed God by
1. blemished sacrifices
2. cheated on tithes
For benefits, the Priesthood corrupted the law with distorted doctrines and heresies and there is no fear of the Lord.
Note - Isn't this depravity similar to Christians in the last days.
2 Tim 3:1-2,4 But know this, that in the last days perilous times will come: For men will be lovers of themselves, lovers of money... haughty, lovers of pleasure rather than lovers of God
In order to save them from this dreadful day, God sent Elijah to bring the Jews back to Him.
Coming of Elijah the Prophet
Luke 1:13,16-17 But the angel said to him, "Do not be afraid, Zacharias, for your prayer is heard; and your wife Elizabeth will bear you a son, and you shall call his name John... And he will turn many of the children of Israel to the Lord their God. He will also go before Him in the spirit and power of Elijah, 'to turn the hearts of the fathers to the children,' and the disobedient to the wisdom of the just, to make ready a people prepared for the Lord."
In the temple of God while Zecharias was offering incense in the Holy Place, the angel told him that his wife would give birth to a son (John the Baptist) with the sprit of Elijah. He was sent to bring love and peace to the families and also to bring them back to the Lord.
Note: Elijah is known as the prophet of fire. The last days' 'false prophet' will bring fire down from heaven to deceive Jews and unlearned Christians.
Rev 13:13 He (false prophet) performs great signs, so that he even makes fire come down from heaven on the earth in the sight of men.
Elijah had already come
Matt 17:11-13 Jesus answered and said to them, "Indeed, Elijah is coming first and will restore all things. But I say to you that Elijah has come already, and they did not know him but did to him whatever they wished. Likewise the Son of Man is also about to suffer at their hands." Then the disciples understood that He spoke to them of John the Baptist. (Read also Mark 9:11-13)
Our Lord Jesus confirmed that John the Baptist was indeed the Elijah mentioned in the book of Malachi.
Warning Many Endtime teachers are wrongly teaching that one of the two witnesses will be Elijah.
That Great and Dreadful Day
Mat 27:24-25 When Pilate saw that he could not prevail at all, but rather that a tumult was rising, he took water and washed his hands before the multitude, saying, "I am innocent of the blood of this just Person. You see to it. " And all the people answered and said, "His blood be on us and on our children."
The Jews condemned themselves for the killing of their Messiah. The death of our Lord Jesus was that great and dreadful Day, when the veil of the temple was torn into two signifying the abolition of His covenant with the Jews and end of Judaism. The resurrection of our Lord started His new covenant of grace.
Note - During this period of grace, all Sabbaths, feasts, sacrifices and law in the OT must be abolished. Only commandments and teachings of our Lord and His Apostles are to be observed.