What does Matthew 6:20 mean?
Christ commanded His disciples and followers not to mindlessly stockpile material possessions (Matthew 6:19). Worldly goods can easily be lost to thieves and natural forces. Every treasure—here meaning money and objects—that can be stockpiled on earth is temporary. Having it comes with no guarantee of keeping it; it's not eternally reliable.Now Jesus tells His followers to focus instead on stockpiling treasures in heaven. These rewards are eternal. They cannot be lost to moths, rust, thieves, or anything else. In financial terminology, they are a far better investment. Christ's meaning here is not that believers ought to have no possessions, or money, at all. The main theme of the Sermon on the Mount (Matthew 5:1–2), so far, has been the priority of motivations. Our hearts and our sincerity towards God matter most of all, so having wealth is not a problem until we start to prioritize it over godliness.
How did Jesus mean for His followers to store up rewards for themselves in heaven? He has spoken a few times in this Sermon on the Mount of acts God will reward. These have included being persecuted for Jesus' sake (Matthew 5:11–12); giving to the needy in secret (Matthew 6:4), praying in secret (Matthew 6:6) and fasting without calling attention to oneself (Matthew 6:18). In short, God rewards His people when they lead lives of sincere devotion and worship to Him.
Rewards from God are more than just experiencing His favor in a moment of this life. Jesus has in mind something other than being allowed into heaven, since salvation cannot be earned by "eternal treasures" (Titus 3:5; Romans 3:28; John 6:28–29). So, what are they? Scripture does not give us an explicit understanding. We know that there "are" rewards, but Scripture doesn't provide any meaningful explanation of what they are. Jesus likely meant His audience to presume that any eternal gift from God would be worth the investment of righteous living on earth.