“Treasures on earth” are material possessions. A rich person would be known by their expensive clothes and jewelry as well as large and expensive homes. A moth (σής) destroys clothing (which is why we use cedar chests and moth balls to keep them away). Sometimes this word is translated as “wood worm” since moths really only destroy clothing, which is not specifically mentioned by Jesus. Rust (βρῶσις) destroys metal, although gold and silver cannot rust (silver can tarnish, but that is different than rust). “Rust” was translated “worm” or “vermin” in some translations (NIV 2011) since it refers to eating or consuming something. A third risk to material possessions is theft. All we are doing by storing up treasure on earth is keeping it for someone to break in and steal. In each of these three examples, material possessions are temporary. Even the most durable treasures like gold or silver will fade away or be stolen by others.
The contrast is with “treasure in heaven.” The things which destroy earthly treasures cannot harm heavenly treasure. Jesus is speaking metaphorically, our good deeds do not generate literal wealth in heaven (and it is certainly not being assembled into a great mansion for you to live in when you get there!) Jesus says our loyalty and behavior should reflect a heavenly perspective, not a temporal earthly perspective. The idea is that all behavior is either for God or not, with no in between ground. You are either storing up treasure in heaven or on earth, and the disciple is to be about the business of storing up the treasures in heaven.
The concept of “treasures in heaven” was common in Jewish thinking, Jesus is using a metaphor that would have been understood immediately by his listeners to mean proper God-honoring behavior.
This is important because our earthly treasure reveals our true loyalties. Jesus says that wherever your treasure is, there your heart is also. What he means is that most people are smart enough to know where they are getting their rewards, and that they will put their effort into the place where they are gaining reward. Salesman make their money by selling, so a good salesman will get to know what kind of person is there to buy and which is there to browse. In a similar fashion the disciple is being told here to know which behaviors and attitudes are worthy of heaven and stick to them.
Scot McKnight draws a contrast between the wealthy and powerful Joseph of Arimathea (Matthew 27:57) and the wealthy young man whom Jesus told to sell all his possessions (Matthew 19:16-30). Joseph was focused on the coming kingdom and used his wealth wisely, the young rich man could not (Sermon on the Mount, 207).
Since Jesus has already warned his disciples they will be persecuted on account of the r testimony, this warning on wealth is especially important. When the Jews were persecuted in Rome, the first phase was loss of possessions. For some of the early Jewish Christians this was enough to drive them away from Christianity and back to Judaism.
Hebrews 10:32-35 indicates some Jewish Christians in Rome had been subjected to insult and confiscation of property. If possessions really indicate the content of one’s heart, then Jesus warns his disciples to not place their confidence in material possessions since those will not endure. These disciples live out this principle in the book of Acts. They live a voluntary life of poverty as they await the return of Jesus. This sometimes involved selling property (land, etc.) in order to support the community (Acts 2:45; 4:32-5:10).
Jesus’s words are perhaps shocking to American Christians (especially at Christmas, when this could be considered treasonous!) Rejection of western materialism is difficult since we like to have “nice things.” Wealth is not inherently evil, but the love of money is indeed the root of all evil.