Malachi - means "My Messenger"
Essentially God says to Israel that he loves her while Israel responds with "no, you don't". Discipline occurs. The priests despise God. They are not giving nor offering their best. They don't even care about doing that. Their heart isn't in it to follow God either.
His covenant with Levi was one of life and peace and now they've broke the covenant. They will lose the priesthood as a result. Levi lost it and it was given to other nations.
2:14 - They are divorcing here. You become adults at age 13 and would have an arranged marriage. That is the "wife of their youth". Israel is dishonoring the marriage covenants when they divorce their wives. She loses her inheritance and is thrown out basically. She is now forced to be a beggar and this practice has become commonplace. They were also using the lame excuse that the woman couldn't have children. It's so fake and disgusting and God sees it. In the same way they do this to their wives, they've done this also to God and basically divorced themselves from God. If they really understood marriage and God's love and covenant, they wouldn't do any of these sick and disgusting things.
"Lord, I send my messenger". Before I come, I send a messenger. [John the Baptist is the Forerunner for the Lord/Messiah. He will witness the evils and bring it to the Lord, "I will be a witness for them". The widow/orphan could not testify in court so he will be the one to bring this to the Lord].
3:16-18 - Book of Life and death (Revelation references it too). God doesn't forget anyone.
3:18 - A return visit is here and in mind (2nd Coming/Last Judgment).
St. John Cassian paraphrased once said "God is always examining our hearts becaues if he can find even one spark he can fuel it and make it a fire for Himself".
There will be a Day of the Lord (the FINAL DAY) where everyone is seperated (wicked and just). Christ's resurrection will grow a remnant.
4:4-6 - I'll send Elijah. St. John the Forerunner is the New Elijah. I will separate myself a new people. John the Baptist starts this with his baptizing in anticipation for the Messiah. Christ's death is a day of the Lord which results in the end of the cycle of death and unfaithfulness in the Old Testament. "Faith is the substance of things hoped for". God's made promises He will keep. John the Baptist has the same spirit as Elijah. Elijah fights Ahab. John deals with Herod. In Christ, we have a new nature. A new creation is to come.