"For the LORD has chosen Zion; he prefers her for his dwelling." Zion is a pre-figurement of the Church, that is you and me. Hence, when we see the Old Testament referring to "Zion" we can consider is referring to us. Psalmist, then, says to us today, "For the LORD has chosen you; he prefers you for his dwelling." The Lord is asking us to allow Him into our hearts as He gives Himself in love. It is up to us to invite Him in and reciprocate this gift.
We do this by storing up our treasures in Heaven. To store up our treasures in heaven means to live a life of self-sacrifice, giving up the treasures of this world to accept nothing but God as the final object of our love. This is most clearly seen when we daily pick up our Cross to follow the Lord and when we love without being loved in return. We people insult us and we don't return their insults; we pick up our Cross. When our children are being brats, but we deal with them patiently and mercifully; we pick up our Cross. When we fast or offer up penances, we pick up our cross turning aside earthly goods to make room for the spiritual goods of Heaven.
It was the opinion of St. Chrysostom the Jesus was using a metaphor for the understanding. When we use our understanding correctly, we are filled with the Truth. We are able to see reality clearly just like the eye is able to see reality clearly when it is filled with light. From this Jesus draws the conclusion that if our understanding is filled with darkness, that is with lies and falsity, then our soul will be filled with darkness. This is because how we act is based on what we understand to be the right way to act. Some people think that it is the point of their lives to store up earthly treasures, and if they believe this then they are never going to want to store up heavenly treasures.
Athaliah, the mother of Ahaziah, is a woman who had this false opinion of the world. She let the eye of her understanding be clouded with the splendor of earthly goods. She becomes ruthless because of this desire and slaughters princes who had the right to the throne and takes all the power of Israel for herself. It's scary how much the lust for power and earthly riches can contort the human heart. But one does have to be as ruthless as Athaliah to be corrupted by the desire for earthly goods. People like to be selfish with their money. They give very little to the Church or to Charities. They're often concerned with maintaining their reputations and will lie and slander others in order to preserve it. We might spend our time being slothful and lazy, so we won't pray or come to Church or be attentive to our wife and children. There are myriads of ways that we put earthly goods before the Lord.
This is why truth is so critical to the Christian life. If we are not filling our intellect with the truth, but instead filling it with lies, we're going to lead ourselves into sin. To prevent this Christ gave us the Sacred Scriptures, Sacred Tradition, and an infallible Magisterium to guarantee that we could know the Truth. He did not want to take any chances. So avail yourselves of these resources! Use them to conform your life to the Cross of Jesus which was holocaust offered to the Lord, sending up billowing incense to our Heavenly Father. Today Christ gave us an important truth: where your treasure is there your heart is. Hang on to that truth and make use of it.