What does Malachi 1:14 mean?

A major factor in Israel's sin is the fact that they have everything necessary to properly honor God. And yet, out of laziness, bitterness, and apathy, they insult God by bringing impure offerings. According to this verse, those who promise God a proper offering, then substitute something less, are actually "cheating" Him. This is certainly immoral, but it's also disrespectful. In modern terms, this is known as a bait-and-switch, where what's delivered is much less than what was promised. Attempting this scam implies that the victim is too weak, stupid, or gullible to be worth fair dealings.

Applied to God, this is unbelievably arrogant. According to verse 10, God would prefer to see no worship at all rather than worship which comes in impure form, or from an impure heart. The attitude expressed here shows another facet of why. Giving God less than He is due, as though He were a sucker to take advantage of, is both immoral and degrading.

As was warned in verse 11, God will be properly honored, with or without Israel. "The nations," as used here, means the Gentiles, or all of the non-Jewish people of the world. Predicting that these non-believing and unclean people will one day respect God, while His own chosen people are insulting Him, is meant to shame Israel's spiritual leaders.