When Jesus talks in Matthew 6:19-21 about “treasures on earth” and describes the earth as the place “where moth and consumption destroy and where thieves break through and steal,” I suspect we think of clothing, eaten by moths, and of money stolen by thieves. (The word I’ve translated “consumption” here doesn’t mean “rust,” for which James 5 uses a different word, but rather means “eating” and in Malachi 3:11 refers to grasshoppers or locusts.)
Interestingly, though, in Isaiah 51, we have similar language, but there it’s applied to *people”: “Do not fear the reproach of men, nor be afraid of their insults, for the moth will eat them up like a garment, and the worm will eat them like wool” (51:7-8).
This destruction coincides with the change in the heavens and the earth: “The heavens will vanish away like smoke, the earth will grow old like a garment, and those who dwell in it will die in like manner” (51:6). These things are temporary but, says Yahweh, “my salvation will be forever, and my righteousness will not be abolished…. My righteousness will be forever, and my salvation from generation to generation” (51:6, 8).
Coming back to Matthew 6: we take Jesus to be talking about rich clothing (which can be eaten by moths) and about gold and silver. And rightly so. But notice that in the earlier context, the hypocrites have their reward in the praise and notice they receive from men, while the righteous are to wait for their reward from God their heavenly Father, who sees their alms, prayers, and fasting in secret (6:1-18).
If we hear Matthew 6 in the light of Isaiah 51, then, it’s not just clothing and gold that Jesus has in view. There is coming a change in the heavens and earth — the coming of God’s kingdom in the near future, in Jesus’ time — and the very men whose praise and notice we’re tempted to seek as our security, our treasure, will be consumed and destroyed. But God’s saving righteousness is forever, and that is the treasure on which we’re to pin our hopes.