What does Malachi 3:14 mean?
Verse 13 described this attitude as "hard against" God. In context, the Hebrew word hoz'qu means "arrogant," so Malachi has already called out Israel for being disrespectful and unfair towards God.The irony of Israel's claim was addressed earlier, in Malachi 2:17. While the people complain about God failing to judge the wicked, they themselves act wickedly! The people disobey the commands of God, then criticize Him when they receive the exact punishment His covenant promised (Deuteronomy 28:15–68). In this verse, the same hypocrisy is on display. The people are treating God with contempt (Malachi 1:7–8; 2:11–14), then wondering why they are not being blessed by Him.
This is not a problem unique to ancient Israel. Modern believers, as well as non-believers, often attack God when they suffer for their own choices. We have a habit, as sinful people, of refusing to follow God, suffering as a result, then hypocritically claiming that there is no benefit to following God. In both cases, the people actually make one valid point. That is, the way they are worshipping God is, in fact, "in vain," since God won't respond to selfish, bitter, insulting forms of worship (Malachi 1:10). The fact that God has ignored their pleas is evidence that their approach to God is, in fact, impure!
The reference to "walking as in mourning" shows that Israel's concept of spirituality is already becoming highly legalistic. As shown in earlier verses, neither the priests nor the people are following God as they should. Going through the motions, numbly rehearsing rituals and sacrifices, is not the same as seeking the face of God. And yet, the people seem to expect God to respond to this charade and bless them.
Note, also, that Israel is making another not-so-subtle dig at God. They ask why they should "keep His charge," meaning they see no benefit to upholding their end of the covenant agreement. Not only does this imply that they are keeping it (they are not) but it implies that God is not making good on His side of the bargain.
This ignorant, arrogant attitude is further described in verse 15.