What does Malachi 3:1 mean?

In Malachi 2:17, Israel throws an arrogant, ignorant accusation at God. This claim is so common, and so vapid, that God Himself is described as "wearied" by it. Specifically, Israel has complained that those who do evil seem to be blessed by God, and then asked "Where is the God of justice?" Israel is comparing themselves to the world, and criticizing God for not dealing more harshly with those they see as sinners.

Given all that the book of Malachi describes, this is an unwise attitude. The priests of Israel have been disobedient (Malachi 1:6–7). The people have been unfaithful (Malachi 2:10–11). And yet, they have the nerve to ask—sarcastically, it seems—where the "God of justice" is. The upcoming verses will show God's response: brace yourselves, because you're going to get what you wished for. Asking God to judge means asking to be judged, as His judgment applies to all people at all times.

This verse specifically promises a messenger, echoing the prophecy of Isaiah 40:3. Jesus will confirm this prediction's fulfillment in the person of John the Baptist (Matthew 11:7–10). Malachi's prophecy is the last word from God given to Israel for four centuries, with John the Baptist being the next prophetic figure to emerge.

Separately, this verse predicts the return of the Lord to His temple. This is not the same person, time, or event as the messenger who "prepares the way," as mentioned in the first part of this verse. This prediction echoes passages such as Ezekiel 43:1–5 and Zechariah 8:3. As such, it is a prediction of the end times.

Notice that Malachi includes two remarks which almost certainly are meant to be sarcastic. God is referred to as one "in whom you delight," and "whom you seek." Given the apathy and disobedience of Israel, this is hardly a compliment. More likely, it's a mocking response to Israel's snide question posed in Malachi 2:17.