I’m going to cheat on these chapters a bit because I think 2:17 is really the start of the next protest. These fellas are wearing the Lord out with their words (their arguments, their bad logic, their blind gas-bagging around, etc.). And, in the same format as the previous disputes, they rebut and ask in what ways they have wearied him (I don’t think these are good-natured, humble pleas of “oh no, we’ve mistakenly wearied the Lord, someone show us what we’ve done!” and more like snarky, incredulous protests that imply the Lord is overreacting.)
God points out that they say ridiculous things like “God thinks evil doers doing evil things is alright, in fact they make him happy” and “where is the God of justice?” In response, God pretty much ignores the first one, it needs no further argument because it’s dumb. However, he elaborates on the protest that there is justice to be done and God is not doing it. He basically acknowledges that they have a right to expect God to distribute justice so he lays out what that will look like.
First, there is a messenger who will prepare the way before Him. Some interesting things to this. First, kings do that kind of thing, send a herald out to tell everyone the king is coming so that they can do road maintenance, trim the beard, wash the kids, tear down any propaganda they may have put up against the king, etc. Also, the implication is that they need to be prepared for the justice/king to arrive (meaning that they are not prepared in the current state). The next verse will show why. Even cooler, the messenger is preparing the way for the Lord to come to His temple (it seems like God hasn’t really come back to this new temple that they built when they came back from exile.) God says it will happen and he’ll send a dude out ahead of him so that you know it when you see it.
v.2 – But who can endure the day of his coming? Ah, that’s why they need preparation. We should remember a similar thing from Amos – the people of God are tucked and ready for the justice of the Lord to come, they call for it. Yet they seem to be under the mistaken impression that it will not also come for them. Like a dude running a refinery who has to crank up the heat to get the slag and other junk off the metals, and like a soap man who manhandles the clothes while he applies the cleaning agent, they better be ready for what they’re asking for. But after the refiner and soap man are down, what comes out the other side is pure, just like the priesthood will be when the Lord returns to his temple. Then the offerings will be good and acceptable like they once were. The God of Justice will come and there will be judgment against a whole host of folks.
This section smells like Jesus because it is Jesus. God does ultimately send a messenger (John the Baptizer) to warn the people on what is coming. And it is indeed a double-edged sword, especially for the teachers and the priests of the day (who have been in view for their foolishness in much of Malachi.) And sure enough, Jesus does return to the temple (brought in by his parents to see Simeon, teaching, throwing tables around, and ultimately tearing it down – that’s some refining). Yes, it’s the same temple they built coming out of exile that Jesus interacts with during his time. Yes, King Herod did some work on it, but same place still. Also note that the NT teaches on each of those things that the Lord will be swift witness to (Jesus touches on most of them himself).
I think the “I the Lord do not change” is in reference to the fact that, even though Israel stinks at keeping the covenant, God is still honoring his end of the bargain by not bailing from the agreement. In fact, he is putting a lot of effort into calling them back into it (again, consistent calls to repentance). His people should have been consumed by now (like the un-chosen Edom will be) but still he calls them back – “Return to me and I will return to you”. But then they argue with him again about whether they are stealing from God. They are, of course, because they aren’t trusting the God with the tithe. They are keeping it or giving God junky stuff and their lack of faithfulness to bringing in the 10% for God’s purposes and promises is resulting in a curse. God says “Try me, aren’t I faithful where you are not?” Most of God’s responses in Malachi have been demonstrations that he is still faithful to them even when they aren’t faithful to him. The tithe is no different. God keeps his promises.
Again a dispute – God says they are using hard words against him, they act like nothing of the sort is happening. It’s getting worse, they are saying it’s worthless to serve God and go through all of these motions we have been going through (trying to do what he says, acting like we care he’s upset, etc.) And then they call for justice upon evildoers who they think are actually alright from a God they are placating with passive, disinterested worship.
But then the tide turns a bit – we’re introduced to a new group, those who actually fear the Lord have gotten together. And a book is kept to log those who fear him, who respond to him and serve him. And what does God do with them? He keeps his promises and takes them to be his treasured position, and he spares them (that’s part of the covenant, the blotting of transgressions). And thus we get clarity between those who serve God and those who don’t.