Malachi is written to the people of Judah after returning from exile in Babylon. They were free from captivity but struggled to rebuild Jerusalem, the Temple and worship practices.

Over four thousand members of priestly and Levitical families returned to Palestine after the exile under the supervision of Zerubbabel and Joshua. Both groups engaged in a variety of activities, including the rebuilding of the walls of Jerusalem, instructing in the law and leading in the religious duties of the nation. However, the priesthood must have deteriorated during this time (especially during Nehemiah’s absence), since a foreigner (Tobiah the Ammonite) was allowed a room in the temple. In fact, the Levites had forsaken the temple for a time. - The IVP Bible Background Commentary: Old Testament

Messenger #1
Prepares the way
(Prevenient grace)

3 A voice cries out: “In the wilderness prepare the way of the Lord, make straight in the desert a highway for our God. 4 Every valley shall be lifted up, and every mountain and hill be made low; the uneven ground shall become level, and the rough places a plain. 5 Then the glory of the Lord shall be revealed, and all people shall see it together, for the mouth of the Lord has spoken.” - Isaiah 40:3–5 (NRSV)

Messenger #2
Purifies the priesthood
(God is making all things new NOT all new things.- Rev. 21)

8 But you have turned aside from the way; you have caused many to stumble by your instruction; you have corrupted the covenant of Levi, says the Lord of hosts, - Malachi 2:8 (NRSV)

10 I also found out that the portions of the Levites had not been given to them; so that the Levites and the singers, who had conducted the service, had gone back to their fields. 11 So I remonstrated with the officials and said, “Why is the house of God forsaken?” And I gathered them together and set them in their stations. - Nehemiah 13:10–11 (NRSV)