5 I'll send you Elijah before the dreadful day of the Lord.
6 And he will make fathers love their children and vice versa,
or I'll smite the earth with a curse.
A few more words about this episode
By Malachi
The book of Malachi is anonymous; "Malachi" is just a transliteration of the Hebrew words for "my messenger."
The author it thought to have been a priest who lived when the second temple was being built. (ca. 520 BCE)
I hated Esau. (1:3)
Why did God hate Esau? Malachi gives no reason, and neither does the story in Genesis (25:24-34). In the New Testament, Paul (Romans 9:11-13) refers to this verse to say that God's love - or hatred - is entirely arbitrary, which is as he thinks it should be.
Whatever Edom builds, I will destroy, because I will be angry with them forever. (1:4)
God will be angry with the Edomites forever, because they are the descendants of Esau (Gen 36:1) and God hates Esau (Mal 1:3).
You've brought wounded, lame, and sick to sacrifice. (1:13)
God continues to demand animal sacrifices. And not just any animals will do. He is insulted when blind, lame, or sick animals are killed for him. (Dt 15:21, Lev 22:19-24)
I'll send you Elijah before the dreadful day of the Lord. (31:1, 4:5)
Jesus said that this prophecy was fulfilled by John the Baptist, whom Jesus said was Elijah - although John the Baptist denied it. (Was John the Baptist Elijah?)
He [Elijah] will make fathers love their children. (4:5)
Then they couldn't be Jesus's disciple. (Luke 14:26)