What does Malachi 3:12 mean?

The end result of Israel's obedience (Malachi 3:10–11) will be prosperity so obvious that the Gentile world will be taken aback. This is an important incentive at this particular point in Israel's history. The Jewish people are rebuilding their homeland, but are still under the control of Babylon (Malachi 1:8). They are spiritually frustrated and bitter over what they see as a lack of justice being done to the wicked people in the world (Malachi 2:17). This verse points out that following God's will is the surest way to restore their fortune and their reputation.

Part of the context of this promise is the Mosaic covenant between Israel and God. The guarantee of blessing, like the threat of punishment, is targeted at Israel in particular, not the world in general. The overall concept of "reaping what you sow" is certainly valid for the Christian believer today (2 Corinthians 9:6–12). However, God does not guarantee material wealth, or health, or success, to those who follow Him. God's true treasures are heavenly (Matthew 6:20), not material.

This verse marks the mid-point of Malachi's third and final message to Israel. The remainder of the book will be a direct condemnation of Israel's blasphemous attitude and a plea from God for His chosen people to turn back to Him.