Where the last chapter ended on a positive note, with the identification that there are indeed groups of people who do fear the Lord, the end of Malachi carries that out with a final reminder to be faithful to what God had commanded and to anticipate the justice that they are asking for (although without a timeline given as to when).
The imagery at the beginning of this chapter is one of anticipation, an oven heating up that will eventually consume the evil. Conversely, a “heat” image of the sun is also used to deal with the righteous, as this sun will risk and bring healing and God’s people will be like excited baby cows leaping out of their stall (who, whilst leaping and making merry, also trample the wicked). All this for the day when the Lord will act.
Now, the New Testament seems to imply that Malachi’s prophecies are fulfilled in Jesus, and in general that seems right. However, only if we are to accept a very figurative interpretation of the beginning of chapter 4, for example, can that be the whole story. Because although it is certain that the life, death and resurrection of Jesus created finality to the judgment situation and, by his example, all those who are not like him are not part of His kingdom, we don’t literally see evildoers turned into stubble and people in an oven. We also don’t see God’s people turned into calves, so it’s probably best to think of this as figuratively dealt with by Jesus yet likely something more physical and tangible at Christ’s final return.
Israel is reminded to remember the laws of Moses, they are good and result in good for his people. Their faithlessness to God by failing to honor these commandments is the root of their issues anyway. He also tells that Elijah the prophet will be sent prior to the great day of the Lord. In their minds, this might actually have been Elijah given that he didn’t die and was just taken up into heaven. However, Jesus says this is John the Baptizer. The next time Moses and Elijah will be paired like this is on the Mount of Transfiguration.
All seems like it will be well and good to end Malachi until the final sentence. The first half is good, father and sons will return to each other (this image likely should be extrapolated to the relationship between God and his people). But if they don’t, God will come and strike the land with a decree of utter destruction. And there you have it.
One of the interesting things about reading Malachi and some of the other minor prophets while being on our side of history where Jesus has already done what He does, is that our understanding of covenant is very similar. Where previous God’s people (or his kingdom if you want to think of it that way) was governed by his commandments and his relationship with the people that serve him. We actually do the same thing, it’s just that our “law” if you will is the living example of the life of Jesus himself. And it’s our relationship with him, which is kept healthy by what Jesus has done as our High Priest, sacrificial lamb, etc. instead of what we do.